My experience is slightly different than the others. My barn is 39'x39' rough dim. The seathing is similar to 'R' panel. One panel is 3' 1.5". With 1.5" overlap the next is 6' 1.5" ect out to 39' 1.5". The corner metal covers 6" on each side, however it needs to come on the opposite side of the ridge, so it will cover 4". So the max dim to the outside of the girts is 39' 9.5". I made it 39' 6".
If you use a different sheathing you will need to do the math for it.