The trailers you are listing will be pretty tight for five if you plan to do more than just sleep in them. Something with a sleeper couch will be a good idea because setting up and breaking down the dinette as a bed gets old. Something in the 24 to 26 range would be nice. Personally, I would buy used for two reasons. First, you can get so much more trailer for the money. Second, you reduce your risk if you find that you don't like the trailer or just don't like camping and you need to sell. If you do like camping, you may want to sell and upgrade.
On length, unless you have storage limitations, there's value in going longer up to a point. Longer seems easier to work with in my opinion, but above a certain length can make the towing setup a bit trickier.
