One other thought -- when choosing a trailer, it is a good idea to buy from a dealer who is not too far away from your home. My closest Fun Finder dealer is an hour away, and it has been a pain to hitch up the trailer and bring it in for warranty work. (Almost all trailers will need warranty work, I think.)
No matter what trailer you get, you will have to be prepared to do some minor work on your own. I personally enjoy that aspect of trailering, but not everyone does.
You asked about length -- it is a trade-off. If you tend to spend a long time in one campground and just hang out in or near the trailer, a big trailer is a good idea -- you can have some privacy and room on a long rainy afternoon, for example.
But if you move around a lot, or spend most of your time outdoors, or you want to fit into remote forest service campgrounds, my advice would be to choose a smaller trailer, if you can stand the claustrophobia. My trailer is ridiculously small -- the box is about 12 feet by 7 feet -- but it is fine for us. Easy to tow, easy to park.
Before making a final selection, spend some time inside the box with your significant other. Life is a series of compromises!
