Size does make a difference in general terms. You can't beat the laws of physics, if you want big full rich sound, you need bigger speakers. That's just general speaker knowledge.
When you try to apply that to actual soundbars sold by manufacturers though (Who the quality of the speakers they're using could vary wildly depending on who you're purchasing from), then other factors apply.
Looking at just 1 aspect though in a vacuum doesn't really help.
I'd head over to AVSForum and look at their soundbar section to get some detailed help.
(Phew, made it through a whole post without saying a single bad thing about soundbars!)