If I was wearing a belly band and had all that room I'd carry a Glock 29, or if I wanted a revolver an SP-101 3 inch with 180s in it would work too. I'd rather have the Glock 29 though.
Front pocket you can fit an S&W 642, or Kahr PM9, but I guess theres nothing to stop a person from putting a j-frame, or PM9 size 9mm in a belly band. Same goes for the different caliber sub-compact Glocks. For example, I can think of a lot of worse choices than a Glock 26 in a belly band.
The oft quoted minimum for defenses is .380 and I don't doubt that people can successfully defend themselves with .380, .32 ACP, .25 ACP, 22 Magnum and even .22 lr. I've just made the personal decision to not carry anything less than .38 Special and the 9mm Parabellum. Some may want nothing less than a .357, .45 ACP, or 10 mm and thats perfectly okay too, if that individual has the skill to effectively use the more powerful handgun cartridges.
"No free man shall ever be debarred the use of arms."- Thomas Jefferson