Instructions
Go to the folder where your "MUGEN" game executable is found. Head to the "data" folder from there.
Launch Windows Notepad from the Start menu. Drag the "mugen.cfg" file to the Notepad window to see the contents.
Click "Edit" from the Notepad menu and go to "Find..." Type "GameWidth" without quotes in the "Find what" text field and click the "Find Next" button to let Notepad highlight one of the display options that you must edit.
Change the number next "GameWidth" equal sign to one of the recommended settings. The config file recommends that you set the width to 640 if you want standard definition, 1280 for high definition, or 1920 if you want full high definition.
Change the "GameHeight" value to 480 if you set the width to 640. Use 720 as the value if your width is 1280; use 1080 if your width is 1920.
Click "File" from the Notepad menu and click "Save" to save the display changes.