Yes, first show us the info to the right of the imported movie when you have it selected in the Project Panel. Second, show us the composition settings. Third, get the info from your media player about the movie and compare. All should be the same frame size. Without knowing what the specifics are it's impossible for me to tell what may be going on.
Did you create your composition by dragging your footage onto the Create New Composition button?
If not, try that.
Otherwise, where did this footage come from? What codec does it use?