I would appreciate your help phrasing the following:
I am looking for elements which/whose/... size/sizes is/are relatively large.
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Whose is the way to go here. Merriam-Webster defines it as follows:
of or relating to whom or which especially as possessor or possessors
Which wouldn't work, because it doesn't indicate possession. It would work, however, if the phrase read:
I am looking for elements which are relatively large (in size).
As to the "size is" vs "sizes are", I would say that both are grammatically correct, though the singular is preferred. The Google stats look as follows:
Indeed, whose is the (only) correct possessive form, for both animate (sentient) and inanimate objects.
The Wikipedia page supports this.
In addition, the possessive version of the non-sentient pronouns is the same as that of who: whose takes this role for all of them. E.g., "I will have to fix the car whose engine I ruined".
The word you need is whose.
Using "whose" in such cases is correct. This thread over at the Daily Writing Tips forum addresses your question: Possessive form of "which".