Enlarge menu fonts?

Hello,

I've installed TeXmacs mainly to provide an interface to FriCAS.  I'm using
a newly updated Arch Linux, and since the Arch package appears to have some
issues, I installed the generic Linux binary package.

It works well except that the menu fonts are very very small - too small,
in fact, to be useful.  (I can just read them with a lot of effort and
squinting).

I'm using KDE Plasma version 6.3.5 with Qt 6.9.0 and Wayland as my
graphics platform.

How can I enlarge the menu fonts?

Thank you all,
Alasdair