Depends on bumper style, swing out carriers, winches, etc. If you want to make sure, with plenty of room up front to clear the tow vehicle on turns, you would need at least a 28', probably better at 30' to make sure you can get the proper weight balance for a tag along. With a gooseneck, 28' of usuable deck space would be sufficient since the rear one could let it's tail hang over the end by 2', though would still recommend 30' for the gooseneck to give a little breathing room.
My gooseneck is a 18' + 5' dove tail for 23' of usuable deck. I can just squeeze my friends CJ-5 and my Scrambler by letting the rear wheels just sit on the back of the dove tail.