The words that you would use must express as closely as possible the intended meaning of the original. So, perhaps, in case you have not done this already, it makes sense to write down the story in Spanish, and think through, very carefully, what meaning each word and phrase need to convey. If you use, in Spanish, 'grandes tetas', it may make sense to say 'large breasts', in English, directly. ('Tits' may come across as uncouth. So, for instance, if the description of the lady is intended to be given through a vulgar remark, then you may consider using vulgar language. But it seems this is not your intended meaning.) If you use an expression that refers to 'grandes tetas' - 'large breasts' indirectly (well-endowed, for instance, as suggested in one of the other answers), this may alter the intended meaning or character of the story. You need to evaluate whether the story in Spanish speaks directly or obliquely, and aim for the same effect in English. More generally, try to understand the artistic style of the original and re-create it in English.
In addition to the words already proposed in the other answers ('well-endowed', 'bust'), there are also 'bosom', 'bosoms', but each of these introduces a slight difference in meaning. So, again, the choice would depend on the intended meaning in Spanish.
See this recent interview with Ann Goldstein, Elena Ferrante's translator. This is translation from Italian into English. Ms. Goldstein discusses three cases that proved a challenge for her: The Hardest Elena Ferrante Lines I’ve Translated
Note the third case, in particular. Ferrante's Italian original reads 'grosse mammelle', which in Italian does mean 'large breasts', and which Goldstein translated directly as 'large breasts'. In the circumstances of Ferrante's story, such direct rendering was entirely appropriate. This is an illustration that simple direct translation is a viable path. (Incidentally, only the words grosse mammelle/large breasts are relevant here; the rest of Goldstein's translation of that sentence can be questioned, however those details are beside the point as applied to the Spanish story.)
Ann Goldstein's translation was published on the 1st of September; it has been widely reviewed (here is an article in NYT; no critic felt that the use of 'large breasts' by the translator was in any way offensive in an of itself. So, the phrase can be used if it carries the meaning conveyed in Spanish.
P.S. Perhaps, 'dugs' found via Google in one of the other answers may require further investigation. The OED seems to think that the word has evolved to be used as contemptuous. If at the time when the story was taking place 'dugs' were free from vulgar connotations but have acquired such a connotation now, it may still be inappropriate to use the word, unless one writes the whole story in an archaic language and makes sure that any vulgar connotations are absent.
dug, n.1
Pronunciation: Brit. pronunciation/dʌɡ/ , U.S. pronunciation/dəɡ/
Etymology: Not known before 16th cent.: origin obscure. Perhaps
radically connected with Swedish
a. The pap or udder of female mammalia; also the teat or
nipple; usually in reference to suckling. As applied to a woman's
breast, now contemptuous.
1530 J. Palsgrave Lesclarcissement 280/1
Tete, pappe, or dugge, a womans brest. 1582 R. Stanyhurst tr. Virgil
First Foure Bookes Æneis i. 14 Her dug with platted gould rybband
girded about her.
1593 W. Shakespeare Venus & Adonis sig. Fiij Like a milch Doe,
whose swelling dugs do ake.
1607 E. Topsell Hist. Foure-footed Beastes 671 The number of young
pigges..I find to be so many as the Sow hath dugges for. a1628 J.
Preston New Covenant
(1630) 477 The promises are full of comfort as a dugge is full of
milke. 1713 W. Derham Physico-theol. iv. xv. 256 With Duggs and
Nipples placed in the most convenient part of the Body of each Animal.
1878 H. M. Stanley Through Dark Continent II. iii. 75 The enormous
dugs which hung down from the bosoms of the women.
Merriam-Webster also appears to suggest that it is vulgar when referred to a woman: Dug in MW
However, both 'sagging' and 'pendulous', suggested in the other answers here, may be suitable, if this is what the Spanish story says.
P.P.S. Some comments after clarifications were added to the question:
It may make sense to use different words/adjectives when the story describes her appearance in her spirit form and when she is presented in the alluring human form, to seduce 'irresponsible' men. The story probably does both. For the desciption of the spirit form, u could use either sagging or pendulous (they have slightly different meanings), or something else; and a different word for breasts - this would depend on the precise nature of her spirit form (in some versions of the story on Wikipedia the spirit form appears to be something close to an animal). If we were looking at the picture while reading the story in Spanish, we may well see the picture in a different light and use different words to describe the spirit form in English. For the description of the human form, if the story highlights the physical beauty and/or sexual attractiveness, it may make sense to say so directly. It seems, the contrast between the two forms may be key. A boader point would be that it is almost impossible to evaluate what the right word is unless we have the Spanish original. We may look at the picture and agree that it is 'sagging breasts', but if we read the Spanish story we may realise that it is 'large taut elongated protrusions, reaching her waist, rested on her torso...' or something else entirely.
P. P. P. S. Further on 'Dugs':
Please note that the word was found and proposed by @David; @Mark Morgan Lloyd recommended an interesting literary source; further helpful observations - from @Fattie.
In light of the discussion around the word (see comments here and @David's answer), I will add some literary examples, to the OED's list above, in case there is something that may help with the Spanish story. The literary usage seems to support the OED's and MW's notes on how the word may have evolved with respect to women. However, as discussed, we do not know enough about the spirit form, and so even though it makes sense to be careful about the word, it should not be ruled out.
- Richard the Third, Shakespeare, William 1593
DUCHESS. Ah, that deceit should steal such gentle shape,
And with a virtuous vizor hide deep vice!
He is my son; ay, and therein my shame;
Yet from my dugs he drew not this deceit.
- Richard the Second, Shakespeare, William 1595
YORK. Thou frantic woman, what dost thou make here?
Shall thy old dugs once more a traitor rear?
- The Faerie Queene, Book 1, Spenser, Edmund 1596
And as she lay upon the durtie ground,
Her huge long taile her den all overspred,
Yet was in knots and many boughtes upwound,
Pointed with mortall sting. Of her there bred° 130
A thousand yong ones, which she dayly fed,
Sucking upon her poisnous dugs, eachone
Of sundry shapes, yet all ill favored:
Soone as that uncouth light upon them shone,
Into her mouth they crept, and suddain all were gone.
- The Anatomy of Melancholy, Burton, Robert 1621
Every lover admires his mistress, though she be very deformed of herself, ill-favoured, wrinkled, pimpled, pale, red, yellow, tanned, tallow-faced, have a swollen juggler's platter face, or a thin, lean, chitty face, have clouds in her face, be crooked, dry, bald, goggle-eyed, blear-eyed, or with staring eyes, she looks like a squissed cat, hold her head still awry, heavy, dull, hollow-eyed, black or yellow about the eyes, or squint-eyed,
sparrow-mouthed, Persian hook-nosed, have a sharp fox nose, a red nose,
China flat, great nose, nare simo patuloque, a nose like a promontory,
gubber-tushed, rotten teeth, black, uneven, brown teeth, beetle browed, a
witch's beard, her breath stink all over the room, her nose drop winter and
summer, with a Bavarian poke under her chin, a sharp chin, lave eared, with a long crane's neck, which stands awry too, pendulis mammis, "her dugs
like two double jugs," or else no dugs, in that other extreme, bloody
fallen fingers, she have filthy, long unpared nails, scabbed hands or
wrists, a tanned skin, a rotten carcass, crooked back, she stoops, is lame,
splay-footed, "as slender in the middle as a cow in the waist," gouty legs,
her ankles hang over her shoes, her feet stink, she breed lice, a mere
changeling, a very monster, an oaf imperfect, her whole complexion savours,
a harsh voice, incondite gesture, vile gait, a vast virago, or an ugly tit,
a slug, a fat fustilugs, a truss, a long lean rawbone, a skeleton, a
sneaker (si qua latent meliora puta), and to thy judgment looks like a
merd in a lantern, whom thou couldst not fancy for a world, but hatest,
loathest, and wouldst have spit in her face, or blow thy nose in her bosom,
remedium amoris to another man, a dowdy, a slut, a scold, a nasty, rank,
rammy, filthy, beastly quean, dishonest peradventure, obscene, base,
beggarly, rude, foolish, untaught, peevish, Irus' daughter, Thersites'
sister, Grobians' scholar, if he love her once, he admires her for all
this, he takes no notice of any such errors, or imperfections of body or
mind, Ipsa haec--delectant, veluti Balbinum Polypus Agnae,; he had
rather have her than any woman in the world. If he were a king, she alone
should be his queen, his empress.
- Translations from Virgil, Pastorals, Dryden, John 1697
Of grass and fodder thou defraud'st the dams,
And of their mothers' dugs the starving lambs.
The goats with strutting dugs shall homeward speed,
And lowing herds secure from lions feed.
"I know thee, Love! in deserts thou wert bred,
And at the dugs of savage tigers fed;
- Gulliver's Travels, Swift, Jonathan 1726
The females were not so large as the males; they had long lank hair on their heads, but none on their faces, nor any thing more than a sort of down on the rest of their bodies, except about the anus and pudenda. The dugs hung between their fore feet, and often reached almost to the ground as they walked. The hair of both sexes was of several colours, brown, red, black, and yellow. Upon the whole, I never beheld, in all my travels, so disagreeable an animal, or one against which I naturally conceived so strong an antipathy.
- Childe Harold's Pilgrimage, Byron, George Gordon 1818
And thou, the thunder-stricken nurse of Rome!
She-wolf! whose brazen-imaged dugs impart
The milk of conquest yet within the dome
- THE RASH CONJURER, Poems, Coleridge, Samuel Taylor 1833
And before 'em their Shepherdess
Lucifer's Dam, 20
Riding astride
On an old black Ram,
With Tartary stirrups, knees up to her chin.
And a sleek chrysom imp to her Dugs muzzled in,--
- The House of the Wolfings W Morris, William 1889
but yet another God they have, and look you! it is a wolf, as if they were of the kin of our brethren; a she-wolf and two man-children at her dugs; wonderful is this.
- Ulysses, Joyce, James 1920
Crouching by a patient cow at daybreak in the lush field, a witch on
her toadstool, her wrinkled fingers quick at the squirting dugs.
- The Waste Land, Eliot, T. S. 1922
Out of the window perilously spread
Her drying combinations touched by the sun’s last rays,
On the divan are piled (at night her bed)
Stockings, slippers, camisoles, and stays.
I Tiresias, old man with wrinkled dugs
Perceived the scene, and foretold the rest—
I too awaited the expected guest.
