What makes the illusion of this special room, called an Ames room, so effective is a carefully chosen vantage point. Viewed from a particular angle, this slanted room looks square, even though the ceiling on one side is actually about twice the height as the other. Since square rooms are the norm in our world, your brain decides the room is square—a decision that causes the people inside to seem to grow and shrink as they move from one side to the other.
This illusion has been used widely to create special effects in movies and television. In The Lord of the Rings, for example, several scenes set in the Shire relied on Ames rooms to make the hobbits look tiny relative to Gandalf. (click diagram to enlarge)