My trailer builder got back to me and asked me if a 45" tongue would be long enough. I had no idea what would be long enough, so I'm going back to the people that know.
I'm planning on having a tongue box and mounting the spare (13") laying down on the tongue. I don't know if that makes too much difference, but how long a tongue should I be looking for for easier backing and side clearance (oh, by the way, my future TD is a 5X10)?