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).