Try Yesterdays Tractor Web Site. They have a lot of specs for previously built tractors. Zimmerer Kubota also has a little information concerning HP and year of manufacturer.
Glad that helped. You can tell pretty well by the bucket size. A 6 ft bucket holds about 3/4 yard and weights about 2200 pounds heaping with sand. 5 ft bucket holds about 1/2 as much and heaping is around 1200# or so. Not sure about 4 ft bucket but probably holds around 700#.