It fits on a 14' trailer no problem. Why have a longer trailer than you need? Yes if you have other things that you are going to haul I understand the reasons for having a longer trailer. But if all you are going to haul is a TJ/LJ a 14' trailer will do it. If you are getting it custom made you can have them put the axles where you need them to make the tongue weight what it should be.
But just like modifying our Jeeps, to each their own.