Well the longer the better.. The longer the bars are the more leverage they apply. Question is are they going ot be used as a tuning aid or a safety item? Mine are basically just along for the ride on the small tire car, well they also keep the car from flipping over. As far as tuning we cant do much with them due to the short length. If they hit to hard with their short length they will unload the tires. It is one of the changes the car will be going through this year. So they are essentially sissy sticks.
On the big tire car we use them as a tuning aid. They are 74" long. We determoned the length by loading the car in the trailer with the golf cart in and that was as long as we could go without removing them.
Here is a picture of the car with the short bars it used to have, 50", this happened a few times.
Here is what we get with the loger bars. We ride the bars a good distance with the car, typically 50-60' and they are a great tuning aid in this car. With the addition of the better rear shocks we usually see a 3-4" wheelie for 40' now. But we can also set it up to play the wheelie game and do it under control.