They’re all set to sizes between 8 and 10 pt, and if the font selector popup window is any indication (I requested 24 pt in it for the screenshot to see that the example text gets scaled properly), the actual fonts are all available:
Besides … the OS selection is what controls the fonts on the top menu (the one going “Datei Bearbeiten Ansicht …” in my previous screenshots), isn’t it? That is the size it should be …
I am lost at this point. As you point out, the problem is in widget rendering specific to LO. There is certainly a setting somewhere. To make sure:
(… quick question, are we talking about LO’s or the OS/KDE theme there? I already tried changing the former from following the WM’s (which is Breeze (dark)) to a couple others, in particular, a freshly installed Oxygen, but that didn’t change anything beyond the icon artwork …)
… The “are you sure you want to restart in safe mode” popup is one of the non-resizeable and thus unusable ones …
… started with --safe-mode from the command line, problem persists (as witnessed by the huge selections/buttons in the first window, and "continue with safe mode"ing and opening the same spreadsheet) … 
Have you tried the flatpak yet? I have the same problem as you and my work-around is to use the flatpak.
Open the KDE store “entdecken”; and go to Settings → Flatpak → Add source Add flathub because I tried the flathub version, I didn’t check the fedora flatpak.
Then, after installing the flathub remote,
Search for Libreoffice and try to install the flatpak version.
(I removed the RPM version, because my desktop links didn’t work when both RPM and Flatpak versions installed)
Let me know if it fixed the problem for you
I think it is an issue to be reported to Fedora…
Could you write the bug report to fedora, or should I report it to them?
I have to admit that it’s been quite a while since I last saw the issue appear. (The laptop tends to run for weeks on end when I’m sitting in the home office, too, so, not many opportunities for it to reappear.) If you can reproduce it now, it’d probably be better if you were to informe Fedora, too.
(No Flatpak, snapd or the likes here. I administer servers mainly, and in that context, we’re not quite convinced of muddying the waters on what versions from whom of whatever software we’re running. :-3 )
