There's three things to consider.
First, where do people own private clubs, once/if they are implemented. So you don't have people lolzoring in the midst of an event. After all, in space it's normal to have ships flying around, perhaps even the odd bump, but if you're in a bar / street and a mob is dry-humping you it might be a bit uncomfortable.
Second, event purpose. Due to the way EVE background works, there's no "RPer" faction. There'll be outlaws, loyalists of different factions, megacorps wanting corp sov etc.
Third, system population and size of rooms. Right now you could cram a hundred people to a chat channel bar/hall/whatever, but when Incarna hits, if you're doing events "in the flesh" instead of, say, a corporate VR channel... prepare to face that "the station administration has placed security measures to limit the amount of people within the operating range of the specific life support units" or however the game will let it be known that the target room is up to capacity. Depending on the size, we'll likely talk about 16 - 64 people or so. The cardboard box that is your first corp office can hold only a handful of people, that "big" bar that took enough resources to make a battleship can host along the lines of 64. My guess.