3) Arena Rules

3a) All battles must take place in a battle box, regardless of weapons being used in the battle.

4mm polycarbonate is the minimum recommended thickness.

3b) The arena will be a raised platform with an area of at least 30 inches (762mm) square.

3c) At least half of the edge of the arena must be unwalled, to allow robots to drop directly into the ditch that surrounds the arena. 50mm is the recommended height for arena walls.

3d) The minimum distance between the edge of the unwalled part of the arena and the battle box (and therefore the width of the ditch) is 140mm.
From the AWS rules.