First and most important thing whether motoring or braking: YOU MUST UNDERSTAND THE BEHAVIOR OF THE LOAD MACHINE. Assuming that you do, then the stop time limit is the shortest stop the machine will tolerate. That covers things like broken gears, broken drive shafts, bent rollers, and how the process material acts when suddenly stopped. For example, on this last one, if the process material is a sheet of steel, will it fold up and jam if stopped too fast. Or, in the case of liquids, if a fast stop launches all the liquid out of the machine and into the basement, that clearly would be too fast.
Again, assuming that you understand where all the limits are, then you need to calculate or at least estimate braking hp. Sometimes, you can get a good estimate of this by watching the acceleration time. For example, if it takes 25hp to accelerate the machine up to max speed in 10 seconds, then you can figure it will take about 50hp to stop in 5 seconds and about 100hp to stop in 2.5 seconds. These will be a bit conservative due to friction losses helping you stop. Lacking any of this, as mentioned above, you will need to take rotating inertia and stop time and figure braking hp.
Don't be supprised if braking hp comes out higher than motoring hp. (I've got one application where the motoring hp is 15 and the braking hp is 75!) If that is your case, you must size the motor and drive for the larger of the two. You can use the short-term rating of the drive which will reduce it's size by about 1/3. You can figure peak short term hp in the motor which will reduce it's size by 1/2.
As for the braking resistor, if you are sure that the only time braking is used is when the resistors are completely cooled off, then you can figure the resistor wattage at 1/10 of the braking wattage. If the braking is repetitive, you may have to up the wattage to the near continuous rating.
Hope this helps.