1.175 or 1.8 depending on brand. 144 up the middle works great for trail. make sure your track is cut for the guards. The guards are designed for a 3/4in track, anything above that will drag, slowing you down and burn gas.
If you run a 1 1/4 track un cut with guards, the track will drag. The guards need to be reduced in height by 1/2 inch, or the track needs to be clipped over the windows by 1/2in.
If you run any stud over 1.18 with stock 9 tooth drivers, you are living on borrowed time. It can work for a day, a month, a year.
The windows on the track are pretty much in line with the tunnel protectors. The lugs need trimmed there so they don't hit the protectors.