Ours is a 21 ft with slide and is the perfect size for me, DW & 2 dachshunds. But like others have said, it all depends on how you plan to use it. On the plus side (for us anyway):
Can easily be towed with 1/2 ton truck
Can fit into some pretty tight spaces, like some at the older National Parks
Easier to maneuver in gas stations and parking lots
Small enough to park in the carport at home
Big enough to stay inside on cold/rainy/windy days
Big enough to have a walkaround bed so no climbing over each other for nocturnal bathroom breaks.
Downside:
Less interior room
Can't carry as much 'stuff'
Not a lot of kitchen storage space
Only sleeps 2 adults and maybe 1 small child