Instructions
Launch Notepad by clicking on the application's shortcut in the Accessories folder in your Start menu.
Click "File" and select "Open." Navigate to the "data" folder inside the directory where you installed "M.U.G.E.N." Double-click the "mugen.cfg" file to open it.
Locate the "[Video]" line in the "mugen.cfg" file.
Edit the numerical values after "Width" and "Height" to change the game's default display resolution.
Save the "mugen.cfg" file by clicking "File" and then selecting "Save."