In the US, or at least in California, I understand you do need a special licence to drive a stretch limo. I think it's decided by passenger seating though, rather than length.
Anything that seats 16 or more (including driver) requires a Commercial Driver's License in the US. Hence the enormous popularity of 15 seat vans with airport shuttle services, school athletic departments, church groups, etc.,etc. Stretch limos around here don't seem to be driven by the cream of society, AFAICT.
Interestingly, campers and motorhomes don't require any kind of special license in the US, even the great big converted tour buses. You can kind of tell by the way they drive...