Considering the Malibu is far from a performance car, I think the difference between 18's and 20's will be negligible in that aspect.
Looks wise, I don't think I've seen a 'Bu with 20's, and assume that it would look pretty gaudy, especially if going for the tacky "bling" style rims.
I'd say 18's or 19's would do the trick, kind of a medium ground between a nice "big enough" look, whilst remaining classy, and offering a good ride and lack of pot-hole damage, that 20's both hamper, and are prone to.