Recorded video changes dimensions while play
Windows Media Player by default will re-size the video to fit the screen.  To get it back down to the actual video resolution you will need to press F11 to exit full screen mode.  

If it is still not the correct size right click on the video and select 100% from the Video sub menu.

As for preventing the video player from resizing the video, I don't think it is possible.

(These instructions are for WMP 12, but should be similar to previous versions)