As a baseline I would ask myself this question:
What is the smallest handgun (size and cartridge wise) that I am willing to get into a gunfight with, against an armed adversary/adversaries?
Yep.
Glock 19 for the win.
That's about where I would place the minimum, too, apart from very rare circumstances where being 100% certain of not printing supersedes this consideration. In those circumstances (less than 1% of the time), I carry something in the Kahr P9, Ruger LC9S Pro, or Glock 43 category, and that still in a belt holster, not pants pocket carry.
When I used to pocket carry, it was almost always a NAA Mini Revolver. There were short stints where I carried a micro .32 ACP auto in my pants pocket instead.
