There are 4 categories of 3 point hitches as laid out by the International Standards Organization. The particular code relating to agricultural wheeled tractors is 730-1 and 730-2. They are labeled as Category 0-4.

Here are the specifications:

Category 0 3 Point Hitch
This category is intended for tractors up to 20 Horsepower
Toplink Pins are 5/8" diameter
Lift Arm Pins are 5/8" diameter

Category 1 3 Point Hitch<- - - Most common
This category is intended for tractors 20 to 45 Horsepower
Toplink Pins are 3/4" diameter
Lift Arm Pins are 7/8" diameter

Category 2 3 Point Hitch
This category is intended for tractors 55-95 Horsepower
Toplink Pins are 1" diameter
Lift Arm Pins are 1 1/8" diameter

Category 3 3 Point Hitch
This category is for tractors above 95 Horsepower
Toplink Pins are 1 1/4" diameter
Lift Arm Pins are 1 7/16" diameter