Kirb,
Big is right, it all depends on the paystation that you are mounting. We have
PDQ Access units, and add 6" to the height that they spell out in their specs because we have a high concentration of SUV's and pickups in upstate NY. It's difficult to hit a height that is good for all makes and models of vehicles, but we tend to like ours a 1/2 course of blocks higher than normal.