Instructions
Click on the "Start" button in your computer's taskbar. Type "Notepad" into the Start menu's search box. Click on the "Notepad" icon when it appears in the search results.
Select the "Open" option from Notepad's "File" heading. Navigate to the folder that contains your installation of "M.U.G.E.N.," using the file explorer window that appears. Open the "Data" folder inside of the main "M.U.G.E.N." folder, and open the "MUGEN.cfg" file.
Locate the width and height values beneath the "[Video]" heading in the MUGEN.cfg file. Erase the current values beside the "Width =" and "Height =" heading, and then type in lower resolutions. If you are unfamiliar with common video dimensions, type in a value of 640 for the width and 480 for the height.
Close Notepad, and click "Save" when prompted.