Yeah, not a "motorcycle" license for smaller bikes. Here in TN anything under "five brake horsepower" is considered a "motor-driven cycle". You can get a license for one at age fourteen.
Limitation on MDC stamp is:
* can't be more than ten (I think) miles from your primary residence.
* cannot drive on any road that is offlimits to a bicycle.
If you have a formal "motorcycle" license, then you can ride one as far as you want, but the highway restriction is still in effect.
The cops require a dealer affidavit on any bike over 125cc that says it doesn't have more than 5 bhp.
I'm not aware of a "minimum" size but they may have one. I'll ask the local goped dealer...