Ninja Eater If your looking for the legal stuff this vary's greatly with the race sponser/class of race. I've seen some that allow up to 75" (with rider sitting on bike). Most are around 60"! Some allow bars, some don't. What cha looking for? A little Kawi Kill Time! Here's a copy of some rules (for example only). http://www.dragbike.com/rock/outlaw_rules.htm
As far as not legal, no clue.
if you go to the ama prostar website, they should have some of the rules depending on what class you're wanting to run. I belive in shootout, there isn't much for length as long as it is street legal. You can also checkout www.dragbike.com to get you all of the sanctioning bodies out there.