This depends on the size of the disc you are ordering and whether or not the fork mountings are 74mm front mounting (barrels pointing forwards) or 51mm ISO mountings (two holes sideways on the forks). The measurements are between the centres of the two mounting holes.
Measure between the centres of the two mounting holes to be sure but whoever is getting the brakes for you should be able to get the right fittings from Hope, as they're good chaps.
AFAIK, the most usual size with Hope is 183mm discs for front / 160mm for rear.
HTH
