[Mr.Shiney]

I think when planning a large event first you need to consider the terrain, time of year, and the types of scenarios you will be employing.

What will be the size of your event staff. What support will they need, and how will the players on the field be controlled.

Once you have made the considerations for the numbers of players, and how they will be separated in to various teams/units. Then I would consider roles or positions on the various teams.