if you're using
psp then warp brush and warp mesh are a good way to go. attached is a before and after (before on the left), but you could easily go the other way and reduce.
like the others said you have to watch how much you try to go, whether enlarging or reducing, as you begin to first stretch textures and then larger, even more obvious factors. you can certainly then correct for some of that but the more you stretch, the more you're going to have to correct other areas.
craig