Both would depend on the position of the camera in relation to the actor. The tighter the shot, the closer you can get the light to the actor, and the less powerful the light will need to be. The farther away the light is, the more power you will need, and probably some way to focus the light to a more limited area. The size of the gun being fired should also be considered.
I have recently come up with a process to create this effect in post which isn't too difficult, for a project I am working on where I had no control over the footage, and had to make do. It involves using the Spotlight Preset
, then adjusting the shape so it basically follows the shape of your actor. It doesn't have to be all that exact though. Then adjust the Glow Color to match the color of your muzzle flash, making sure to use a fairly greyed out version of the color you want. Through some composite blend mode trickery it actually does a fairly decent job of simulating light being cast into the subject, though its still not as good as practical lights.