The Nurtured Family and MOE inserts both come in two sizes - one for small FB's, and one for all the larger sizes. Mediums, Larges, Petites, and Extra Larges all take the same size insert. Seems weird to me, but that's how it is.
For now all I have are MOE inserts. They seem to be working nicely so far. I'm anal about pilling on my FB's, and I like that the microfiber inserts don't cause additional pilling when washed with the fleece like natural fibers do. Of course, I'm already thinking of trying out some other types of diapers, so my pristine, FB's only stash could well be infiltrated by natural fibers soon, and I just don't see myself doing seperate laundry if that happens...
I know that the WonderFulls inserts will fit for all sizes of FB's. And I think that Joey Bunz come in at least three sizes, but I'm not 100% sure about that. You could always stuff with flats or prefolds - those could be folded to fit any size perfectly.