Ah grasshopper, first you must paint fence. Haha. Showcrop is correct, the draft is the downward force on the hitch. As chain length gets longer, the angle is reduced and the downward component on the hitch is reduced because the horizontal component increases. In other words, it takes less power to pull the sled as the chain length increases.
If you want to learn a lot in a short amount of time, check out the Antique Tractor Pull Guide. There is a whole chapter dedicated to hitch physics. See link below.
What kind of tractor are you pulling? I personally love the light classes and have pulled an Allis CA, Massey Harris Mustang and MM BF at various times at 3000#. I'm rounding up parts right now to build something unique...