Since I am going to have to keep at least one of my computers running XFCE (it doesn’t need remote access) because MTP works better on Debian 9.5/XFCE for now, I needed to look at how to set menu and application font sizes. There is actually not a GUI to do this. There was not an issue with this on serverpc up till now, but changing the display around (rotating) has for some reason shrunk the font sizes on the Whiskers menu and in Gimp which is really annoying. Qgis also has an issue on this computer with tiny check boxes next to layers, which may be affected by similar settings.
It’s a bit hard to understand but in the Settings menu if you go into Appearance, then into Fonts, the answer is to set the Custom DPI setting. The default is 96 I think, but in my case I screwed this up to 150, with the default font set to Carlito 9. It’s important to note that the default font setting only affects the panel and title bars and other miscellaneous stuff i.e. it does not impact on the menus and other bits of application UIs. In this case adjusting the setting way higher than it already was fixed the issue.
This all seems a bit strange as the small menu and app text only started today and I have never had to adjust this setting before now but that’s how it works.