I'm in agreement with those who are saying small enough to comfortably carry, and I also want something that is stout enough to shoot comfortably. A .45-70 Govt derringer meets the first criterion, but I doubt I'd log much practice time with it after I fired it once.
I also want a full, 4-finger grip. None of the subcompacts I handled met that criterion, although I understand there are pinkie extensions for some. I put rubber base pads on my compact mags to meet this criterion. I must admit that I found concealing a full-size pistol did seem quite doable, but I live in the Pacific Northwet where are attire makes this fairly easy 11 mo of the year.
Regarding reliability, while important I think it's not as important as the gun being easy and comfy to carry. Most DGUs involve drawing, but not firing, the weapon. Thus, having a gun is far more important than having one that is super reliable.
Regarding practice, John Lott's examination of FBI crime stats discount the relative importance of practice, training, and experience. Even unpracticed, untrained, and inexperienced handgun owners make good use of guns. Practice is important; just not as important as having the gun available.