Each drive will have a minimum acceptable ohm value for its braking function. This sets the maximum current thru the drive and thru the voltage-sensing or "chopper" circuit. You can use higher resistance and get less braking torque but that spec is the minimum.
The braking wattage can be different due to a lot of things. The maximum wattage would be the drive hp rating converted to watts. That would be the rare situation when the drive is braking at full rated hp continuously or for more than, say, one-half hour.
You can get by with lower wattage ratings on the resistor if the energy being wasted thru the resistor is less than continuous. The lower the duty cycle the lower the wattage can be for the same full braking torque. This is due to the fact that a resistor is a pretty forgiving device and will take heavy over-wattage hits if allowed to cool between hits. The maximum over-wattage ratio that I use is 10/1 and that only if the resistor is cold (room temp) every time the braking is done and the braking doesn't last more than about 20 seconds.
Us can also get away with a lower wattage resistor if you are only doing a little braking. There is no rule that says that, for example, on a 100hp drive, you have to do all your braking at 100hp! If all you need is a 10hp brake on a 100hp drive, size the resistor accordingly.