My wife's '02 3.0 is in need of new tires... the Michelin 255/55/18 have about 30% thread left.

Question: can I get different size tires without investing in new rims? Stock tires are 255/55/18. For example, would it be possible to stagger? Should I stay stock with 255/55/18 all around?

Advise please.

Thanks!