dont think it really matters what width you make it,we did add 4" then 6"and then the big banger8",i think now you make it to suit your buggys dimensions,remembering it has to fit on the trailer and the trees and gate posts get horribly closer,,,,
Your right the only real consideration is the trailer as it is 2m wide and the wheels fit on at the front by 25mm either side. It can be widened easily to suit the front end width. I just don't know what width beam I have to order to replace mine as I don't know whether mine is a 4, 6 or 8 over standard because I don't know what standard is. If anyone know the standard width it would be great.
I will post a pic as soon as I can figure out how.