i have the small pink one....i can fit miss buffey in there fine but she does not have a whole lot of room to stand up and turn around...she fits fine laying down though...and she is 3.5 lbs...but she has pretty long legs and a long body (length = 11inches) so maybe this is why?
so if you think your baby might get to 8-10lbs, i would definitely get the larger one!