How to Choose an External Regenerative Resistor
The more common name of this resistor is "BRAKING RESISTOR" , it is commonly used on most AC drives that doesn't support feeding regenerative energy back to the Grid.

As for calculating the power requirement for the resistor is depends on the kinetic energy which is calculated from the mechanical characteristics of the system ( acceleration , deceleration , inertia ..) 

There are a lot of resources for example :

PowerFlex Dynamic Braking Resistor Calculator

frizlen Online calculator


[PDF]Application Note # 5448 - Galil Motion Control