On Mac OS 10.14, the default size of the Serial Monitor window cannot be changed via editing ~/Library⁩/⁨Arduino15⁩/preferences.txt. The current default size and also the minimum size are considerably larger than would be appropriate, especially on smaller Screens (such as the 13" MBP).

This should be editable according to the settings for the editor window, with commands such as:

serial.window.height.default= ...
serial.window.height.min= ...

etc.