Are you talking about aspect ratio? I do not know why you would want that, but take a look here:
Tools -> Options -> Playback -> Video -> DirectShow video playback settings...
Aspect Ratio - it is set to "Source Aspect Ratio" by default. That means it automatically adjust to an aspect ratio appropriate to the video you are playing. You can try setting it to a specific ratio, such as "1.78:1 (Widescreen)". Note that the value you choose here is used only when the setting is not found for a specific video file. You can set the aspect ratio of a specific file by using right-click menu while the file is being played. If you do that, the setting is saved for that file only in the database. In other words, besides the global default setting, you can change the individual setting for each file.