Plasma screns, LCD monitors, Projection Screens, Interactive Whiteboards and Copyboards are measured diagonally across the screen. The diagonal is quoted in inches.
The aspect ratio is the image width divided by its height.
Projector screens generally come in 3 different screen format sizes: square format (1:1 aspect ratio), rectangle or video format (4:3 aspect ratio) and widescreen and HDTV format (16:9 aspect ratio).