All graphical development tools are built using the XPCE graphics library for SWI-Prolog. This system defines symbolic fontnames. For the tools two fonts are important. The fixed font is used for running text in editors and the normal font is used for pretty much everything else.
To change anything, you have to create and adjust a personal XPCE
Defaults file. The easiest way is to launch the XPCE console using the
And then select File/Edit preferences/XPCE user defaults. If you do not have such a file it will ask to create it and copy a default template to the new file. Now you are ready for editing.
Changing size globally (Windows)
Near the bottom of the default template you find
You change the scaling factor for all fonts by removing the comment character (!) and change the value 1.4. Bigger values give bigger fonts.
Changing specific fonts
Alternatively you may uncomment the entire display.system_fonts block and change one or more of the definitions. You can use the application File/Demo programs and then FontViewer to see what fonts are by default available. You can also mount any font available in the system using a system-specific font identifier. This is totally different for X11 and MS-Windows and you are requested to consult the XPCE User Guide for details, specifically Specifying fonts and Accessing Windows Fonts.