There's no information about this in the hard coded limits thread, so I thought I'd play around with it. I tried a number of values, starting with 63 and progressing all the way to 2^32. That last value should have caused an overflow in a 32-bit unsigned integer, but the game managed to handle it. Apparently there is no effective limit to the value you can enter, but at what point a larger value stops making a difference I can't say.

A brief experiment showed that a value of 63 will enable a unit to fight to the death without breaking, so I doubt a larger value would matter. I haven't looked into whether a smaller value would have the same effect.