The big isn't too big unless you're really petite. I'm 5'5'' and 110lbs and it looks fine on me. IRL, it doesn't look so big as it does on the NM BV model for some reason b/c it slouches once it gets broken in and when you have things in it. And the large one can't really be used as a diaper sized bag b/c the bottom has no width and also isn't flat, so it really doesn't fit as much as you'd think. (unless you want the side to pop out and not look slouch and akward). It fits less than the city w/out looking weird.
I like the look of the small one on the edges b/c it looks a bit rounder for some reason, but it'd be too small for me and I wouldn't want to constantly have to hold the bag w/my hand. Small can't go over the shoulder.
There are only two sizes: small ($1260) and the med/large ($1630) and no in-betweens. Hope you find your perfect size!