The copula verb "být" / "to be" does not have a direct object in accusative, it links the subject "we" with a noun predicate that is in the nominative case. Later you will see that it can also be in the instrumental case.
"My jsme velcí muži i velké ženy" is marked wrong. I guess it's because of the "i". So, we cannot use "i" here, only "a"? I have to confess I thought the i / a thing was more or less like in Russian but I see it's not. What's the difference, then? Thanks in advance!