Originally Posted by WoadWunner
2" is industry standard
That is more or less correct.
A class 1 hitch is just a flat piece of metal, that has a small locking pin and is rated to only pull about 500 lbs. Think of a "K" car pulling a jet ski. These hitches were more or less phased out about the time the last "K" car rolled of the line as well.
A class 2 hitch is 1 1/4 inches square and is rated to roughly 1500 - 2000lbs. It is still availabe on many cars and smaller SUVs.
Class 3, 4 and 5 receivers are all 2 inch square, and have varrying weight ratings based on how beefy they are and the tow ratings of the vehicles they are attached to.
Just figured I'd pass on a little trivia since I was late in answering your actual question.