How long can a vex pneumatic piston be?
I am assuming that you are referring to the high school competition Vex? As you probably know there is a 18x18 inch area limit for the robot. The starting position must remain within this limit but there is no limit to how far it can reach, whether or not it becomes unstable and breaks is a different matter. I haven't used pneumatics in 2 years so I forget the default length but Length shouldn't be a big factor to be concerned with.