To answer the 2nd part of your question.......For the most part, it is the same size cabin as for two. There is either a sofa bed that pulls out or a pullman that comes down from the ceiling or wall with a ladder. The same holds true for a quad. On rare occasions there be an odd shaped room that might be slightly larger, but they are not really advertised.
Here is a website that gives cabin size:
<http://www.cruisestateroom.com/ships.htm> There is a feature on the homepage to also compare ships.