My Ifor 510 trailer measures 6ft 8.5" between the breast bar and the breech bar, when they are set at the maximum distance apart. This is too big a distance for my 15.3hh HW maxicob, who takes 6ft6ins rugs, so I move them closer together. He still has plenty of room front and back. Hope this helps.
ETA remember rugs measure from mid point of chest, not the furthest forward point.