Warren,
I bought the 20' from Dean Sloan and it works great with the Viper. You could go the 18' but if you speak with Dean he will advise going ahead and getting the extra 2'(and NOT because of additional money in his pocket
Look hard at getting the wind diffuser in the front...really saves the Viper fascia when traveling and the winch he offers is really handy. The trailer have lockable tool boxes,deck lights and electric brakes. He has made some changes and tells me that it is much easier to unlock-back-tilt-pull forward-lock than the earlier versions.
Great guy to deal with and a great open trailer.
No Flame Intended...Just Observation,
CStegall