The notion of a carry gun is very difficult to pin down. I discount this as a "duty" question, which automatically ups the ante to something that's just not comfortable. Now, to many people "comfort" sounds like a sissy concept- but it it ain't comfortable, sooner or later you're going to take it off when you shouldn't or not have it with you that one time....and you know the rest. You think it doesn't apply to you, but it does, baby, it does. That means concealable AND comfortable.
So what's "comfortable?" Since I live in a non-carry state, I've never been able to deal with that question personally, but I did deal with it with two people. One was a friend in a CCW state who wrestled with this problem endlessly, tried a half-dozen different guns. The gun he actually ended up carrying was a Beretta Bobcat .25 ACP. Although he tried larger guns (he also tried the Tomcat, in .32, as well as all the others), he discovered that the tiny Beretta was the only one he could wear and forget about.
The second guy is a cop, and his concern, as he has been given more investigative duties, was a fairly concealable gun that still had respectable stopping power. "Respectable," in this case, and this is supported by some authorities on the subject, he took to be .380 (9mm Kurz). After reviewing the choices, he decided that the only acceptable candidate was the current manufacture Walther PPK stainless.
{Note: I realize the whole idea of "small" is relative. I also think the criteria presented here are relevant, and the solutions, for the individuals involved, are reasonable. I also realize there are people on this forum who could make a .454 Casull seem reasonable as a "-small concealed carry gun." Nevertheless.}