Shouldn't be too difficult to calculate.... First you'll need to know the rotational inertia of the spindle; with that and the rpm you calculate the rotational energy in Joules. Then you decide how quickly you want to stop it in seconds which will give you a figure in Joules/sec i.e. watts. Then size your resistor using watts = V2/R where V is the back emf of the motor. If you're slowing it down quickly you probably don't need to have a resistor that's continuously rated at the power, more like the peak power rating.