Well I guessed wrong, my 16 HP has the mule setup you described and three blades. I will have that unit off the tractor this week to sharpen the blades and can measure the pulleys at that time.
In the mean time I did measure the pulleys on the smaller tractor deck with two blades. The idler pulley setup is similar in that one pulley is fixed and the other pulley is used to tighten the belt once the deck is in place. For the two blade deck the fixed pulley is 3.25" diameter and the tensioner pulley is 3.50" diameter. All measurements are the inner flat diameter that the back side of the belt runs against. I replaced the fixed pulley (it has a TSC price tag on it) so I can't say if it is the same size as the orginal. However, in answer to your question I believe if you get close to the size everything should work since the system uses the spring loaded tensioner pulley.
Let me know if you want the sizes of the three blade deck pulleys.