If you're playing on Windows and seeing how the Mortal Kombat games are DOSBox you can press Alt+Enter.
You can also use GOG's DOSBox Configurator (sometimes called Graphic Mode Setup in the Start menu) and check "Full Screen" under Basic Settings. If you you see any black bars on the top or bottom, go into Advanced Settings and check "Keep Aspect Ratio". Don't forget to save settings. I attached screenshots.
Alternatively, and the method I prefer, just open dosboxMortalKombat3.conf (or similar for Mortal Kombat 1 or 2) in notepad and change the lines...
fullscreen=true
aspect=true
and save the file.
Hopes this helps.