Check the weight capacity of your 3 point hitch and work down to a safe comfort factor from there. Do a google search on weights and measures - they have tables that tell what different materials weigh ie concrete. Personally I've never seen a need for a ballast box - I just put on my box blade which has an extra 250 lbs of steel weight attached to it. Usually a decently weighted implement on the back works fine (and I don't have loaded tires)
The weights are like the ones used on the fronts of big tractors, must be 70-80 pounds each. They just set on the bar. The bar is blue, think it must be a NH part. Planning on adding a trailer hitch to the bar and have 4 weights on each side of the hitch.