In compositions, a distinction is made between inner and outer size. The outer size is the part of the composition that is visible on the main stage or the frame. This is defined in the layout inspector of the main stage.

The inner size is specified in the composition by the page properties within the composition. Furthermore, you can specify here whether or not the composition is scrolled vertically or horizontally. This step is also possible via the composition section of the Layout Inspector on the main stage

If scrolling is enabled for a composition, it can scroll to the size of the exterior. (See  Page and Content Size )