Hi rsth. Welcome to the forum. I can't tell you the size for sure, but 32 mm is common. However, if you're doing brake work, that nut does not get removed to remove the rotors. I just wanted to mention that the torque on that nut is critical and is very high. If you place the vehicle's weight on the wheel with that nut loose, you will permanently damage the wheel bearing. It will be noisy after that and sound like an airplane engine.
Let me know if you have a " captive" rotor that DOES require that axle nut to be removed. If that's the case, I'll delete my reply so the other guys will see your question and post a response. They might post picture and instructions too. I don't have access to the service information.