I'm in Minn, looking for somewhere to ride around Connecticut. I've got a 65" buggy, and I've seen a lot of trails in NH labeled 50", and found one crude map saying all trails in southern NH were 50" (with , of course, no delineation of "southern").
Most of the web sites for trails that I've found have been very economical with information. Very few will offer a map, other than an occasional offer to sell a paper one. Most won't even say what knd of trail they have, just "trails". I'm accustomed to separate trails for horses, hikers, etc., so I don't know what to make of the area (VT, NH, ME). I found two areas in northern NH that alluded to 65" vehicles, Jericho Mntn. and Great North Woods, but neither spelled out which trails, nor how many miles of trails, allowed 65". Jericho Mntn appeared to be mostly state forest/park, and Great North Woods mostly private (logging Co.).
As best I can tell, every trail in NH is closed for "mud season", some till the end of May, and possibly beyond.