As a player I am playing in the Dungeon of the Mad Mage adventure. On page 11, it describes how to determine ceiling height for rooms:
A room's ceiling is at least as high as the room is wide, often higher. If a room's ceiling isn't specified in the text, assume it's the minimum height.
What dimension do we use to figure this out? For example: a room is 30 feet (N-S) by 40 feet (E-W), with a door on the North wall and a door on the West wall.
Would this room be 30 feet or 40 feet "wide" (and thus 30 feet tall)? Assume the room height is not otherwise specified.
What if the room is odd shaped?