I've heard that size dosent matter.....I've been told that the best gun is the one you have on you. While I respect all firearms...I personally wouldn't carry anything smaller than a compact/mid sized (Glock 19/H&K USP Compact size) handgun.
With that being said I often don't carry unless I'm driving near a shady area. So I suppose a pocket pistol is better than nothing.
Seriously, if you unload six shots into someone's chest, I don't think it matters much if it's a 9mm or 380.