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 slightly preferred. The Google stats look as follows:

• "are * whose sizes are" — 94,700
• "are * whose size is" — 1,020,000

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:

• "are * whose sizes are" — 94,700
• "are * whose size is" — 1,020,000

Searching the British National Corpus returns four results for "whose size is" (one of which is actually used with a plural noun, "segments whose size is"), but none for "whose sizes are".