You won't go wrong with the 3500 pound one. Now if you mean diameter, you'll just have to measure and see.
When I hook yup my trailer, I set the tongue down on the ball, latch it. Then pick up the truck and the trailer to connect the weight distribution bars. Usually connect the chains and bars just by hand, no cheater pole required. Then let it all down again. I recommend 3500 pound jack if you connect this way, the easy way. Do the opposite to unhook.
2012 Ford F250 ext. Cab 4x4
2002 Jeep Wrangler Sahara
2008 Yamaha V-Star 650 Classic
2008 Work and Play 18LT