I have a 2001 Disco, and am planning to put a 2" OME heavy duty spring kit on it. What is the largest tire size I could get away with, with still having plenty of room for articulation, and tight steering offroad.

 

265/75/16, 32" tire will fit well. It will cost you in gas mileage, like around a mile and a half a gallon.
A lot of people go to a 245/75/16 if the are just doing minimal wheeling.
Do you have a CDL t/case or have you checked yet?