In the FWIW dept:

Most house cars with horizontal brake wheels were built before ~1930, so I thought it might be helpful to review the relevant Safety Appliances standards that prevailed at that time.  They were pretty bare bones:

1.  wheel dia at least 15", preferably 16"
2.  staff centerline 17-22" from car-end CL
3.  at least 4" clearance from any surface around the wheel itself.
4.  brake safely operable while car in motion
5.  brake step optional

That's it.  I can't find any minimum or maximum (or typical) height above roof or running board.  It looks like a car-by-car decision.

I did not look at flat cars, gons, or hoppers.
--
Dave Parker
Swall Meadows, CA