Large but even the mediums you could put the light on the side as it does not stick out past the sealing lip.
Another interesting thing is the light is magnetic and there must me some steel in the rubber molding as that is all thats holding the light up. I'm going to run it to see if it stays put. If not it comes with sticky back Velcro.
I like the quick removability of the light in case I need to use it somewhere other than inside the pannier. Simply pluck it out and illuminate whatever I need too.