Look closely on top of the coupler. There should be some info scratched onto the coupler, including something that looks like 2 5/16".
If there is no info on the coupler (the part of the tongue that fits over the ball), then go to Wal-Mart and buy a 2 + 5/16ths inch ball. Take that 2 5/16th ball to the trailer and see if it fits. If its too big for the coupler, then Wal-Mart will give you a full refund. Then buy a 2-inch ball.
My TT with GVWR of 5,600 has a 2 5/16th coupler. My utility trailer with GVWR of 5,000 pounds has coupler for a 2" ball. So I'll bet your trailer has a coupler for a 2 5/16th inch ball,