It all depends on speed, but 6 feet is probably gonna be too short. What are you using it for? Summer setup or winter setup? How tall is your setup? What tricks do you want to be able to do on it? (if you reply with some of that I can give a better estimate, but..) For a summer setup, anywhere from 10-25 feet is good. On snow, 20-30 feet is better since you have more speed. However, the longest PVC you can buy at Lowe's or Home Depot is 10 feet, so my advice would be to make a 10 foot rail first and see how you like it. If you want more time on the rail, make another 10 foot rail and put them end to end for a 20 footer. If you have 2 10 footers you can also play around making elbow rails, A frame rails, down flat rails etc. so I'd recommend doing that.