macOS-specific
Game only fills up a quarter of the window on macOS on retina displays Right click on the MewnBase app -> Get Info -> and turn on "Low Resolution Mode". I'll try to see if I can fix this soon so that it's enabled by default.
macOS version just hangs on a blank screen on launch
This usually seems like an issue with macOS's security quarantining. Moving the app into another folder sometimes seems to help. You can also install and run the game through the Itch Desktop Client, which won't have this issue.