Under the circumstances, the change to the Document Windows size is indicative that the Preview panel is undocked, a floating palette that can easily be affected by values in the Preview Dimensions.  The best remedy is to constrain the palette by properly docking it to the IU.  

In terms of launching Poser completely to factory settings, you need to make this selection under 2 tabs, Document and Interface, in General Preferences.  Otherwise, you may not see any changes.

@romangirl: if any of this makes sense, you should be able to navigate through Poser and reset the proper parameters to prevent this from happening in the future.  If you don't understand what you are attempting to do, please acknowledge that you don't know and allow us to guide you through the process.