Be careful! The shank is also sized for the weight you will be towing. You can find many different size shankes for a 2" ball (which is probably the size you need for your trailer). However, you want to make sure it can withstand the force of you entire boat/trailer weight with supplies. If you get one that isn't strong enough, you can actually rip the ball from the shank. You'll loose your trailer and the ball will still be in the coupler.
Just read the weights in the desciption and get the correct one - even if it means buying a new bar for the receiver because the hole is too small for the correct size.