Here's a link (pdf) to the current Greenbrier Plate F 100t Boxcar, which I think fits the OP description.
I'd say the wheels are 36" - if anyone wants to confirm it by measure the scale diagram in that link, be my guest.
The size of the wheels depends of the load capacity of the freight car. Most box cars carry under 70 tons of payload - 33" wheels will suffice. Usually 77 tons of load is the point where you want to go to 36" wheels. Actually it is more the size of the axle bearing. The wheel diameter is increased to make a larger 'contact patch' with the rail. They work together so that the rail is not deformed by the increased load.
Jim
Modeling BNSF and Milwaukee Road in SW Wisconsin
BIG ONES!
Hi,
What size wheels does a 52' high cube box car take?
Thanks
Chris