Unfortunately I don't use ffdshow myself, so I don't know if ffdshow has (e.g.) a possibility to resize video-screen or any other feature which could possibly cause this.
I would suggest to try a different video "Rendering mode"
(under rightclick > Options > Preferences > Video > [Video rendering])
Maybe also check if ffdshow is indeed taking care of showing the video.
You can do that under (rightclick >) Options > Filters > Advanced
(You must have loaded a video!)