For teaching (and for my eyes on a 4K screen), I need sometimes Eclipse with larger icons and UI fonts (the editor fonts can be easily adjusted within the Eclipse preferences).

What works for me (with X11) as described in https://linux-blog.anracom.com/2024/08/28/eclipse-too-small-icons-on-high-dpi-screens-workaround-for-kde-plasma-by-scaling-on-x11-and-wayland/

• In the KDE display settings (at the very bottom of the configuration dialogue): set Global scale to 125% or 150%.
• (Re-)Start eclipse via

   GDK_DPI_SCALE=0.5 GDK_SCALE=2 ./eclipse &

I also have in my home directory a .gtkrc-2.0 file and need to find out whether deleting it, would change anything -- some applies for the gnome-tweaks command.