I like a 203mm rotor. It's torque-y and the right side of too big to draw too much attention or become an incidental impact liability. 220mms can be overpowered for a light rider on a light bike and you can get wheel-locking issues.
The original owner and converter didn't clamp the motor in place...
I went about rectifying that and realised the next problem.
The bike has an oversize bottom bracket tube, that needed to be slieved to accept the motor spindle. Original owner did that with pressed roller bearings.
Ok so the Bafang locknut couldn't bite the frame because of the larger BB ID it was pressed into the cage of the roller bearing. This meant if I do clamp the motor it would break loose smashing into the brake line on the downtube, and pincing the hose to disable the rear brake...
The fix the owner used was to not lock the motor and when you depowered it it would fall back down on the BB and the brakes would start working again. So simple! Fantastic!
This is what the brake hose looked like after 500km of that craic...
I'm impressed how robust they are.
This is a torque arm required situation.
Custom fabrication, using the original Bafang locknut, with an inner tube frame protecting, damper slieve.
See how that works?
...I don't understand how it should warrant a lower spec by that logic...besides I use the rear for steering more often than stopping.