The pin size depends and the "work load" of the clevis. On the other side of the one you have pictured it has a stamp on it, something like 8T, or 12T (T stands for ton (2000 lbs.)). This is the rated work load for that particular clevis. People seem to think they need some huge clevis, but when you look at the rating it is like 12T (Do you have a 24 thousand pound truck?) So If he has a standard Toyota or something of the like you should be sufficient with say a 7T or 8T rated clevis. The main thing with these is that you don't pull sideways on them, they are not designed to be pulled on the threads. I know I got a little to "techy" on this but...
Kevin
