Is the composition preview resolution set to 1/4? If so, switch it to Full resolution.
Are you using File/Export to output your movie? (Sounds like you are, because many options use the current settings, like Preview Resolution, when exporting.)
If so, try using the Composition/Make Movie function and use the Render Queue for output, which will give you many more options and controls for your output.
The first thing to do is to check the movie properties by opening it in the player. Make sure that it is actually the size you want.
If this movie was a quicktime movie and you opened it in Quicktime then scaled down the QT window and saved the file you've added a header to the movie that says you want to open the movie at the new size. AE sees this tag and uses it to define the composition size.
The fix is to open the movie in the QuickTime player, view it at actual size, then save the movie. Now when you import the movie into AE the size tag will match the pixel dimensions of the movie and everything should work just fine.
Nevermind I got it.