How many squares is this rectangle on the Clue board?
Movement occurs between rooms, not across them. As soon as you reach a room, the movement phase ends. As soon as you start to move, you must leave the room to do so.

The rules don't address this directly, but there are clear implications that a room is considered a single location (not a square, but similar) when it comes to movement. For example:


  It is possible that your opponents might block any and all doors and trap you in a room. If this happens, you must wait for someone to move and unblock a door so you can leave!


Also,


  You may not forfeit a turn to remain in a particular Room.


If a room constitutes of multiple squares, it's possible to move your character without leaving the room. It seems to me that this is not in the spirit of the game.