What kind of movie file are you trying to create?
Are you using the Render Queue?
If so, are you enabling the "Stretch" feature in the Output Module (which, you should probably almost never do)?
The solution depends on the output format and the pixel aspect ratio of your composition. Unless you are using one of the non square presets (NTSC D1 for example) the PAR should always be set to square pixels. Many codecs have strict size requirements. Either of these could be causing you problems.
Just as a test I'd render out an Animation codec QuickTime at this size to see if that works. If is doesn't then you've set up something weird in the output module or render settings.