RVs are divided into three different kind based on their size from small to large:
Class B – About 17-21 feet, usually sleep 2-4 people.
, Class C – About 20-32 sleeps from 4-10 people.
Class A – Bus or Coach usually longer than 30 feet sleeps 4-10 people.
Please refer to the following http://www.TripAdvisor.com article for a more detailed overview of the various kind of RVs: