Explore the forums for opinions on size.
I did extensively, and ultimately decided on a used '27 Safari rear twin. With the whole family in tow, we convert the front gaucho to a queen with an air bed and let the kids (15 girl & 18 boy) share the rear. Otherwise it is the front queen with an early morning/late night rear lounge for the restless.
Only drawback to length is parking that sucker, otherwise on the road with the appropriate rig, not much of a difference. State and National facilities, as well as some private campgrounds, do often have length restrictions though. It is all part of the fun of researching the correct unit.
Any "extra" space comes in handy. Sometimes the longer units represent a better value per square foot. Just make sure they are structurally sound.
Enjoy the experience and hit the road!
'00 Safari 27' Rear Twin
'03 Ford F-250 7.3TD/CC/LB 4x2 Bio-Diesel
"Hmmm... Smells like French Fries!"
/ WBCCI FCU 8579