Setting frame size
The Composition window in After Effects contains the viewing frame and an area outside the frame that you can use to move layers into and out of the frame. For your convenience, After Effects includes a variety of presets, each containing settings for frame size, pixel aspect ratio, and frame rate. You can also create a custom preset and save it for later use.
Because After Effects can position footage items outside the frame, specify a frame size no larger than the actual size of the final viewing screen. After Effects previews and renders only the footage items within the frame. (See "Working with higher image resolutions" on page 53.)
The viewing frame occupies the center of the frame's pixel area, which is centered in a larger work area. You can position items outside the frame if, for example, you want a layer to enter the frame from one side, cross the screen, and leave the frame on the other side.
