The length of the TT could make a difference depending on how tight you have to turn. As mentioned, if you have to turn your wheel to lock to make the turn. Then the length of the TT could determin whether or not the tongue will hit your bumper. A longer TT will track farther inside your turning radius than a shorter TT will. You could do it easily with an 18' TT vs. a 30' TT.
It doesn't matter. 15 foot or 40 foot they all have a 4 foot tongue on them. Go out in an empty parking lot with your trailer hitched up and go to lock and drive around in circles. I know of no vehicle that will turn so sharp that the trailer will whack the TV. Backing up is a different story.
The only thing that will matter is the swing radius. That will be worse on a long trailer with a long overhang and it could take out things like street lights or fire hydrants or things like that.