The only limit is the frame. If you don't use fenders you could probably shoehorn a 45mm in there (I have a T800). Do you need to? Probably not.

A more reasonable size would be anything from a 28mm to a 37mm. 37mm was standard on mine. You should, however, decrease the tire pressure with width. A 28mm can be safely pumped up to around 110psi. A 37mm shouldn't be pump up past 90psi - if that. I've had blowouts on 37mm at or above 90psi. If you happen to be a couple of hundred miles from the nearest bike shop, you could be walking a looooong way