Poetry writes its own rules. But if I saw "wax'd" in the same line with "withal," I would not be inclined to interpret "withal" in the one non-archaic sense that survives today (which Merriam-Webster's gives as "together with this: besides"). Rather I would suppose that it was being used in one of two archaic senses—either to mean "therewith" or to mean "nevertheless."
Other readers might associate it with the Shakespearean meaning "with," as in Rosalind's line in As You Like It:
I’ll tell you who time ambles withal, who time trots withal, who time gallops withal, and who he stands still withal.
Merriam-Webster's specifically observes that "withal" in this sense must be "used postpositively with a relative or interrogative pronoun as object"—which is why when Orlando asks "I prithee, who doth he trot withal?" Rosalind answers "Marry, he trots hard with [not "withal"] a young maid between the contract of her marriage and the day it is solemnized"—but I wouldn't count on my readers' understanding that fine point of usage.
If your goal is to sound oldish but not altogether archaic in this poem, I think you might do better with "beside" (in place of "withal"), though you'd have to move it after "wax'd" if you wanted to retain the line's meter:
He wax'd beside in stature great.
...but this maneuver creates complications of its own, since "beside" (as a preposition) meaning "besides" (as a preposition) is modern English, but "beside" (as an adverb) meaning "besides" (as an adverb) is archaic, according to Merriam-Webster's.
If you don't mind sounding up-to-date, you could simply (and clearly) end your original line with "besides" or "as well" in place of "withal."