4-low, 2nd gear. wheel size doesn't really matter, tire size does though. if you are stock size you should be fine. Stock tires won't have enough traction to break anything other than traction.
Personally I've tried both ways taken it easy and gettin the sled rollin and building boost and just mashing the pedal. I did better just mashing the pedal becasue I got the front of the sled off of the ground and got the weight of the sled on the tires of the sled and got my speed up quicker and made it alot further that way.
And like abobb said if it starts hopping quit right away. I havent seen anybody break anything unless they let it start hopping then stuff starts flyin.
It is alot of fun to do. Once you do it you'll want to do it again and again.