Well, there are pygmy sunfish (Elassoma species); these are not true sunfish, but look like them. They are very small- about guppy-sized, and may be kept in small tanks (10 gallons or less).
The smallest true sunfish are the Enneacanthus species, including blackbanded, blue-spotted, and banded sunfish; all are usually under 3" long, and a pair or trio may be kept in a 20 gallon.
The larger sunfishes are in the genus Lepomis (these are the species you are most likely to encounter); the smallest of these are bantam, orange-spotted, and dollar sunfish (3-5" long). A pair or trio of these could be kept in a 29 gallon tank, or a larger group in a bigger tank.
If you have a 40-55 gallon tank, you could keep medium-sized species like longear, spotted, and red-spotted sunfish; these usually max out at 6-8".
The biggest sunfishes, like pumpkinseeds, bluegill, redears, greens, and warmouths, can reach 10-12" in length and need a substantial tank, probably a 75 gallon at minimum.
You probably won't be able to find all of these where you live. Next time you go fishing, take photos of some of the sunfish you catch and post them up here. They can be tricky to ID, especially when small, but I'll do my best! Better yet, post them at the NANFA forum; I know the guys there will be able to ID them.