Question

Is it my feet (isn’t) big or my feet (ain’t) big ?

Answer 1

depends on the dialect of where you are from, but the gramaticly correct sentence should read:

my feet aren't (are not) big.

Answer 2

My feet ain't big

My feet (isn't) big would be incorrect.

"Isn't" should not be used for plural nouns such as feet. It would be used for foot.

My feet (ain't) big would be correct because it is a term that can be used for plural nouns.