Here are some pics of the trailer I built over winter, maybe you can get some ideas from it. I used 8" I-beams from an old trailer house, and 3" channel for the crossmembers. It's decked with 2x10 treated lumber. It's 21' long (18'+3' tail). Some features I added that I've been happy with are 2 1/2" pockets with a 2 1/4" -1/4" thick tie down rail on the whole outside, a 2" receiver on the front for an attachment point for winching up dead vehicles, and backup lights on the fenders. I also got some 15k lb. d-rings at TSC and welded them on the corners.
I used 2 6000lb axles (one w/brakes), and I built 2' tall sides for hauling wood. You can fit 2 full cords in it pretty easily, I was hauling 4 facecords in the pic b/c I didn't want to overwork the truck too badly.
I still need to do some work on finishing the ramps. They work well and flip over nicely, but I want to build a "wedge" on the top so when I flip them up I have a full 21' of flat deck which is why I haven't painted them yet. I also may swap out the non-brake axle for one with brakes, and also get a bigger truck.
For you tractor guys out there the tractor is a 1964 Ford 4000 that was bought new by my grandfather. I restored it for my dad last summer and was going to get a disk to run over a food plot. My wood hauling helper is standing in the foreground of one of the pics also.
Overall it pulls and works well, I would have done some things differently, but I was on a strict budget and did the best with what I had available.