I ordered the 18-20ft item.
My trailer is 21ft. This cover actually had enough room leftover to cover the rack and generator i have mounted on the tongue. Actually, thats awesome in my case but if i didnt have the generator, there would have been about 3ft of extra material bunched up in the fron or rear. Other than that, im pretty happy with my purchase. My wife and i had it on in about 20 mins.
Directions say to turn it inside out and roll it up, then unroll from top of trailer. Instead, i opened it, and just layed the front over, my wife pulled it while i let the rear fall to the bumper. It was way easier than getting it all dirty by following the instructions. I think i paid $159 for it and i really feel good about this purchase (no regrets) so far. It also has a lp cover but we didnt need it due to the large size, it covers the whole front. Also comes with material for patching. If youre considering this purchase, go for it, i think youll be happy.