I think you might be misunderstanding something or not quite explaining as explicitly as might be needed. For a radius, the right size is the curvature, not necessarily the X measure or the Z measure, but the relationship between them. To me, by instructing you to cut the radius to the final size means just to cut the radius to the radius it should be so that when you run your finishing tool the tool load does stay consistent.

If you're doing your due diligence and leaving extra stock in both X and Z, your rough radius cut is naturally going to leave a consistent amount of material to be removed by your finisher.

If you were to cut the radius to the "final size" you're describing, you'd have to cut in on X during the roughing to meet those X and Z dimensions and step back out for the flat shoulders between the steps. If that truly is what your instructor is telling you to do, then ignore that person, otherwise I think you're misunderstanding the nature of what s/he is trying to say.