It's like a house, you fill the space you have. LOL
We just went from a 30 foot Coachmen to a 40 foot Eagle that will be delivered to our seasonal site in the spring. I know we're going to love the larger space but at the same time I fear we'll fill it up just as badly as the smaller space.
What I'm trying to say is consider what you'll be using the trailer for. The larger it is the more tow vehicle you need but at the same time you want to be comfortable, have room for guests, etc.
At some point during the consideration process you'll find the size of trailer that balances out your needs and wants, even if there are a couple of small sacrifices.
2010 Jayco Eagle 351RLSA
2011 Nissan Titan 4x4 (don't worry, I don't pull the Eagle with it!)
Currently shopping for a Silverado 3500 Duramax
Oh, and I have a tent... Somewhere around here....