Wilsons Sandbox

Hello all-I inherited a motorcycle trailer from a brother in law. Problem is that the rails are only about 4 inches wide. I want to have new rails made and would like to get opinions from others on what a good width would be. My XR350 has tires almost 6" across so I am thinking about going to a 7 inch width. Does anyone out there have a trailer for dirt bikes and could measure the rail width? Thanks.