Hello Franz,
With the XFCE font settings, on Qubes-OS 4.1b1 the access path is :
*Q menu* > *System Tools* > *Appearance* > on the opened window, choose
the *Fonts* tab, then change the *Default Font* size (example: from 10
to 14).
On Qubes-OS 4.0.4, the access path is near of the above path.