Sorry for the delayed response, I've been out of town with work and not a lot of extra time.
On my trailers I knew where I wanted to mount the trailing arm and I knew where I wanted the axle center to be so I made the arm fit. I'm no expert but I don't think it really matters how long they are. I would think a little longer would be better that a little short. The bushings I got from energy suspension, it was a kit with bolts and bushings. I told AT what bolt pattern I wanted for the trailers and I ordered the spindles, brakes, shocks, airbags and limiting straps from them. Along with lights, stoves and battery chargers. Hope this helps.