There are no hard and fast rules when it comes to an ideal size but nothing smaller than 2.35in, even on a lightweight short-travel, trail bike. The maximum size you can run on a trail bike is limited by the amount of frame clearance you have although anything over 2.5 is probably overkill. Obviously, always check for clearance and any flex in the rear wheel – if the tire rubs on the stays under load it can wear through the paint or worse.