The largest I' ve ever seen was on a Locust that was about 3 foot across....true story.
OK let me explain...it had a section on the side of it that stuck out ...kind of a hump on the one side...about 8" wide(kinda shaped like a lightbulb...round on the back and the front jutted out). It was rubbed big time. The same year...there were 3 cedars that were 10" or so that were rubbed also, in the same general area. We still haven' t seen the buck that owned those rubs and that was 5 seasons ago....he' s probably gone by now.

As far as " usual" big rubs, I see them on cedars....and they usually are 5-7" . Never seen a big one on an oak in my areas.