Hey, I'm new to working on bikes and am about to try to build my first back wheel with this set up;

Shimano FH-M565 Rear Hub
Mavic XM317 MTB Rim
DT Swiss Competition DB Silver Spokes

My main issue is that I don't know which length of spoke to order. Any help with this would be brilliant. Also, any opinions on the set up are welcome but I am restricted by cost a bit and I only go off road occasionally.

Thanks

Oh, and the wheel is 26".