Do you have at least one of the step pulleys? I wasn't sure from your posting. If you don't and would be okay with 4 steps, I can measure one of the pulleys on my Bridgeport.
Otherwise, it's like this:
The step pulley arrangement is basically two cones oriented opposite each other. For each "pair" of pulley steps, you want the following distance to be the same, which represents the "unwound length" of the V-belt you'll want to use:
(pi * (radius of a step of pulley A )) +
(pi * (radius of the corresponding step of pulley B)) +
(centerline distance between the driving and the driven shafts) =
The belt's "unwound length".
D = belt's unwound length
rA = radius of pulley A
rB = radius of pulley B
cD = distance between shaft centerlines
(pi * rA) + (pi * rB) + cD = D
For each step, you want to keep this length (D) the same so you don't need to adjust the motor back and forth.
You could also just divide up the difference in size from your smallest to largest pulley in 5 steps and seek a pair of pulleys with those sizes. (Ok actually, it's just the 3 steps in between.) That would be 3 pulleys to figure out - one at halfway between, one halfway between the middle pulley and the smallest, and one halfway between the middle pulley and the largest. Right?
Of course, you could also just try to find a pair of matching 5 step pulleys that'll fit the shafts, then find a belt to fit. That's probably what I'd do...
My $.02 worth of mud...
Science. If you don't understand it, don't talk to me about how it's going to end the world.