Try this
Instead of a bridle, use two lines. One from each towing eye, to a cleat on either side of the transom. This makes it less likely for a single line to part, and loose the dink. As to line size, I use 3/8 on my 8' Merc. dink, and could more than likely get away with 1/4 or 5/16. I tow mine when inside, and running around all the little islands here. But if going offshore, or for longer distances put it on deck. Mine rolls up and stows in front of the mast nicely. If your like me, and tend to get absent minded sometimes, pull it up tight to the transom. You can get it right up against the boat, and can even raise the bow of the dink up some. I prefer to do it like that for a couple reasons. Keeps the dink from filling with water if it gets snotty, and keeps you from getting the painter tangled up in the prop.