I think this is something to do with GTK settings but the font sizes used for Firefox, Chromium, Calibre (that I’ve seen so far) keep changing.
I’m running Tumbleweed with daily updates and 4 days ago things were fine. 3 days ago Firefox etc went into microscopic (to a 72-year old) font mode. The next set of updates 2 days ago returned font sizes to readable and yesterday’s updates went back to micro-fonts.
What’s going on here?
Same here. KDE 5.27 switched to xsettingsd and the GTK envs seem to be gone. Furthermore, user home .Xresources is ignored for GTK. Now, the latter is pretty standard Gnome “we know better than you” dev thinking. If your X is set for any other dpi than 96, check this:
My X is set for 144 dpi and that works for everything except GTK. When you do a “dump_xsettings”, you will probably see that dpi there is set to 96. In fact, it is set to 98304 because for reasons unexplained they use dpi values multiplied by 1024. Anyway, copy the output from “dump_xsettings” to ~/.xsettingsd, modify the dpi values to match actual X dpi, logout, login, everything back to normal. I also added a line “Xft/DPI 147456” just to be sure.
If your X already runs at 96 dpi or you are using Wayland, you probably need to check elsewhere.
And it carries on, next day (Friday) back to normal size then Sat/Sun a return to micro fonts.
I’m not changing any settings (or at least not intentionally) - I’ll try reporting a bug
KDE font sizes are currently a terrible mess. What worked for years is now broken. On one machine I get different behaviour dependent of graphical login of different users.
Needless to say, it’s just my user that’s borked.
I really hate having to re-create a user setup I’ve spent ages crafting to my requirements. If the user is borked, I can’t just copy across all the settings so, it’s working out just which ones to copy to the new user, and you know you will always miss at least one …
Here’s the latest - Amongst all the other things I’ve tried (i.e. editing any file in ~/.config with ‘gtk’ in it) I went back to play with KDE System Settings and tried assigning other GTK themes. Somewhere down the line I’ve got back to normal-sized fonts again. Looking for modified files with find .config -mtime -2 throws up everything in the gtk-3.0 & gtk-4.0 folders. I’ve made copies of them so, if the fonts go back to minimised, I’ll see what changed and report it.
I’m pretty sure I’ve tried selecting other GTK themes before but maybe I had a ‘senior moment’ and didn’t apply them - at 72, I’m allowed a senior moment 
I suspended the machine to RAM and resumed a few hours later: Fonts are back to their original correct size!
Logging out from KDE and logging in again reproducibly shrinks browser font sizes. Killing kded5 restores correct sizes.
I created a new user with an empty home directory. Its graphical login is affected too by tiny browser font sizes. Killing its kded5 restored the original font sizes.
Up to now one user was never affected by this annoyance. But when I double checked again while typing this the other user has the small fonts too.
I presume the tiny fonts are caused by a broken kded5 or one of the applications it manages.
I was wrong, it’s not just one user 
A possible fix I found while digging around is to set $GDK_SCALE and/or $GDK_DPI_SCALE.
For the moment I’ve added
export GDK_SCALE=1.5
to ~/.bashrc and it seems to be maintaining normal sized fonts - at least for Firefox & Chromium.
From HiDPI - ArchWiki
Note: Plasma 5.27 dropped use of GDK_SCALE/GDK_DPI_SCALE variables and switched to Xsettingsd. It should be installed to make scaling work for GTK apps. Or you can set this variables manually as described in #GDK 3 (GTK 3).
That’s where I found the info - couldn’t remember where I got it from
The desktop that’s giving me the problem will be retired shortly and I’ve copied the user’s homes onto the notebook that will be replacing it and everything is fine there so there’s something not right in the desktop’s system.
I did try a forced re-install of various kde/gtk packages but that didn’t cure it.
At this point I’ve just got too many other things to sort out, so I’ll live with my ‘fix’ on the desktop until the notebook goes active.
Thanks for your help 
Snapshot 20230306 fixed the issue of non-uniform font sizes. After rebooting into 20230306 all applications displayed the small fonts observed previously only with the browsers.
Uniform previous font sizes were restored by selecting "System Settings > Display and Monitor > Display Configuration > Global Scale > 200%.