Go as long as you can afford. Tell the dealer what you are hauling. Trailers are sprung for heavier loads than we use with model airplanes. If you can get a lighter spring package, do it. I get more damage from the bouncing around in the trailer than I do from flying. Realize, everthing will have to be tied down securely. If you intend to store it in a garage, check heights and clearance before you buy. Most trailers will not fit into a standard garage door.
Make sure you have vents. It gets REAL hot inside in the summer. I prefer dual doors. I light package is nice, dark in there also. Make sure you have a wheel for the tongue jack. About all I can think of right now.

Oh, and check with your housing neighborhood association. Most don't allow trailers of any kind to stay in driveways. I'm storing a friends at my house now for that reason.

steve