The factory sized OEM tyre was roughly equivalent to a 120/90 x 17, but everybody fitted a 130/90x17 as soon as they could, for that bit extra rubber and tyre life. I've tried a 140, but only once, and most 140s will rub the swingarm.
Otoh, the good news is that radial rears in adventure treads (not knobblies) are available and the bike responds really well to them, cornering as if on rails. Those in 130/80x17 are a natural match for it, and closely suit the original factory gearing too, so no real downside.