You can adjust the full MM body border into the custom.css file.
(/MagicMirror/css/custom.css)

like that (play with the values until you’re happy) :

body { margin: 20px; height: calc(100% - 40px); width: calc(100% - 40px); }

You should maybe also check the settings of your screen itself (depending if you have that issue outside MM or not).