AFAIK, none of the states have licensing requirement based on size... doesnt' matter if the boat is 13' or 130'.
At the Federal ./ USCG level, there are no licensing requirements based on size for privately operated vessel.
Insurance, as others have mentioned, is another issue. Whether or not you will be able go get coverage depends on your experience, location, boat type, value and whatever guidelines the underwriter uses.
Often, having a OUPV/6 pack, will be looked favorably by the underwriter and that may help.
As to the USCG training and licensing (6 Pack or Master), it's a joke. The experience requirements dont' make a lot of sense since it's based on commercial vessel needs and not adapted to the "yachting" needs.
for instance, for the 6 pack and 100T, required experience is based on nr of days at sea. The definition of a day, 4 hours underway, means that the typical boat owner may find themselves unable to log most of the experience.... someone who'd go to bimini every month for the week end woulnd't be able to log any day because it woudl be less than 4 hours per day.
yet, a guy with a 13' whaler who sits jsut outside the jetty drift fishing for 4 hours can log a day...
and you don't even have to operate the boat to log the time... if you're just crewing, you can log the time.
Then when you try to get over 100T, the requirement becomes 8 hours underway per day... very few boats (except fishing) will be underway for 8 hours a day...
doens't make a lot of sense.... The result is that as Angela says there are many paper captains out there that none of us would trust with our vessels.