Since I haven't known how I'm going to print them (and I have never actually printed a photopass picture, sigh), I literally save each end-edit in each size. I don't want to make an awesome black and white, focused in, with a border and a sticker picture, save it in 4x6 then want it in 5x7. Because when I go to print that 4x6, zoomed in, stickered, picture in 5x7 it's GOING to cut some of it off. And that won't make me happy.
So I save each edit I like in every size.
Now if you aren't going to add borders or stickers or anything; if you aren't going to zoom in at all or change the orientation, then you'll probably be safe just saving it in one size. I don't; I still save the plain pictures in each size as well. But then I don't have photoshop or any other picture editing software, so I'm kind of stuck with what I have from them.