"Are they big?"
No, something it is missing, some noun in nominative, like "Jsou to veliké země." But that is a different sentence.
I believe the -ý ending in velký is meant for singular (masculine) nouns. And jsou is plural not singular.
So in your explanation to DanielleSh528752 You're saying there HAS to be a nominative subject in order to put "to"? So "To jsou velici chlapci" would work here. But as it is, without a specific subject indicated, it's simply "Jsou velici (or velika, velike)"?