Lower resistance = faster voltage drop after power removal, but more heat generated.
Higher resistance = slower voltage drop after power removal, but less heat generated.

Once you have drives attached to the power supply, they'll help reduce the voltage. The bleed resistor is more a safety measure for if there are no drives attached, as otherwise the capacitor(s) could retain quite a high voltage for a considerable amount of time.
I've just checked what I've used previously, and for my digitiser (around 40VDC IIRC) it's a 270ohm 15W, and the mill a 680ohm 25W (around 55VDC IIRC).