for the screen size, look in the magicmirror doc for the electron options, you’ll want the full screen option probably. but you can set width and height too. You make these changes in your magicmirror config.js
When messing with modules and stuff, I generally set my screen size width and height to about half of my monitor so I can edit config and things comfortable on the same screen.
tips:
Ctrl+m will minimize electron and thus magic mirror, if you need to get to the desktop and are running full screen.
click on teh electron instance then ctrl+r will reload magic mirror. That way you don’t have to completely stop and reload when you are messing with css or config. At least 90% of the time ctrl+r wll be enough.