Originally Posted by CRUM
137mm is what you want. But you will have to cut off the excess. The way I do it is this. Measure the hub from axle nut to axle nut. Write that figure down. Then Measure the thickness of the dropouts. Add both both the hub figure and both dropout figres together, deduct 3mm, and that is the length. Seems to work for me everytime. One thing though, when adding the thickness of the dropouts, always add both into the equation. One time I didn't and, well, I had to start over.
Also, if you're lazy and enjoy doing things half-assed (like me), you could just put the axle in the dropouts, line it up so the same amount hangs out on either side, and then mark and cut.