It might not always be possible to auto detect the correct font size for scaling he UI from the screen size and DPI data provided by the hardware. For example on a Rasp.Pi using analog composite input there is not data available. To allow the developer to configure the correct size a system property to override the default is needed. The developer can already set in CSS for example:

.root {
   -fx-font-size: 24px;
}

but it would be nice to have a command line option so a binary build of application can be used on more than one hardware setup with out rebuilding which may not be possible.