When dealing with backward facing step problems, What are "wall units"? How do they relate the grid spacings?

No such terms in general textbooks. You may consult specific references.

Probably, when you measure the length of the inverse flow zone in units of height of the breakaway.

I guess you are talking about Y+ values. If so, due to de-attachment, there is no meaningfull definition when dealing with steps. I would use same boundary layer mesh as for the incomming flow. For in-core flow I would define the mesh based on re-atachment length or mixing layer thickness (depending on flow behavior) Hope this helps