The longer the bars the more leverage they have. You planning on using them as a tuning tool or a safety device? Lots of stuff to learn if you plan to use them as a tuning device for sure. If you truly use them in this way 1/4" and inch the wrogn way can lead to very bad results. I'll show you in a bit.

As for how long. HP plays a rol as well as rear suspension set up weight bias, shocks and how well they control things and so on. This is how I did it with the big tire car. We loaded the car in the trailer as far forward as I was comfortable with. Shut the rear door and measured what was left. Viola we had our bar length, in my case 74". I sure you know we use them as a tuning aide on both cars. I tend to ride the bars for a fair amount.

Back to the 1/4" deal. In this video and picture my crew chief had the bars set incorrectly. 1/4" of stagger on the wrong side lead to this. This is midflight Oh yeah this was with a set of 50" bars on the car. Which do not provide near enough leverage on my car. We went to the long bars but bent the uppers three times before we finally buolt a set of double wall slip tube uppers. Like I said we tended to ride them pretty hard back then. We now have a much better pair of custom built rear shocks and can calm it down a bunch.


Here is the video version
http://www.youtube.com/watch?v=PhaW9zfEkZE&feature=plcp