Lazy, hazy proof:
An isomorphism is, among other things, a bijection. So all one needs to do is show 2 things:
1) A linear injection preserves linear independence
2) A linear surjection preserves spanning
These two facts together show that the image under our given isomorphism of a basis for the first vector space is a basis for the second space, and since the isomorphism is bijective, they have the same cardinality.