I do understand that -á is used for neut. pl. (see AnCatDubh's comment), yet I dont see where in this sentence it is stated that it should be neuter. If I would add a noun in front of the sentence (or if I was talking to somebody about), e.g. zeny, then it would be feminine and therefore would have to end in -é (Vidis zeny? - Jsou velké? //Do you see the women? Are they big?)
-> I've studied Czech in a bilingual school for many years, yet havent spoken it in about 10. I know I forgot a lot, but this sentence seems "wrong" to me.
It's stated with the very ending -á. That's how we know that it's about something neuter. If it was "Jsou velké?", you could just as well ask - where in the sentence it's stated that it should be feminine? etc.
"jsou velká" "jsou velké" and "jsou velcí" are all equal translations of "are they big" and since we don't know the context, any of these is right