I'd say 2.4-2.5 measured knob to knob. Or tires advertised as 2.3, 2.35, and some 2.4s. Schwalbe tires were generally oversized even before the wider rim movement. The Magic Mary measures closer to 2.5 knob to knob even though it's advertised as 2.35. I wouldn't put anything like Maxxis WTs or tires with profiles specifically made for 30mm and up rims though.
I have a bike with 25mm rims and I'm running 2.3 Maxxis tires. They measure just about 2.3 knob to knob. Non-WT Maxxis tires are undersized by the way.