You need to re-download the RGSS301.dll that comes pre-packaged within the game. The reason this is happening is most likely because you have been modding/viewing the game through RPG Maker VX Ace (as I do), and the default .dll for most games is set to an AR of 800x600.
To fix this, you must either:
Re-download the .dll and play only this game until you are finished with it
Re-download the .dll, save it somewhere in its own file, and whenever you want to replay this game, place the downloaded copy into your Data folder (not recommended)
Re-download the .dll, and place it in your RPG Maker folder (either through Steam or elsewhere), thereby making it the default AR (recommended if you plan on modding)