For what it's worth: I've been racing most of the winter on a 92" (51x15) in the 3/4 field in Colorado Springs. I'm the smallest gear in the field by A LOT. One of my friends was talking about gearing down to a 98" for a points race.
In the P,1,2,3 races, I've used a 92" (fine) and a 94" (SHELLED). Depends on who shows up, and how froggy the field feels.