I look at it from this perspective:
What guns am I willing to get into a gunfight with?
Generally speaking those are not little mouse guns. The smallest I am willing to go is a Smith 442 J frame, and that very rarely is a primary arm. Usually it is a BUG for work. Normally the smallest I am willing to go and that I shoot well is a Glock 26 9mm.
That being said, I have found that it is an extremely rare situation that a Glock 26 can be carried, that a Glock 19 cannot. So, for the majority of my needs, a Glock 19 is the smallest day to day tool that I shoot very well and I am willing to duke it out with X amount of adversaries with.
While the G26 and 442 make great BUGs, the G19 is the choice when the chips are down and I need to makee hits fast under stress.