Trailer length question

Hello all, long time lurker here with a dilema. I am in the design process of a gen ben but I have a bit of a snag. I want to design the trailer with a receiver in the back for my dirt bike and a platform in the front for the wife's four wheeler. I figure for the deck of the trailer with the fourwheeler and a 8' benroy is only 12' and should be fine, then I second guess myself. I will be building the trailer myself. Any ideas?