Since I installed i3 on Arch Linux I noticed a too big font size (in what was the first thing I saw, the i3bar at the bottom), so I changed the font size in the .config/i3/config file (now the line reads font pango:DejaVuSansMono Nerd Font Mono 5.
Soon enough I noticed that the size of the menu bars of the application was too big, so I edited it with Lxappearance (now the default font is DejaVuSansMono Nerd Font Book with size 4) and the GUI of Lxappearance itself looks perfect.
But the rest still does not, as you can see from the screenshot: the font is too big, I'd say huge, in the floating Open window, wherease it is tiny, if not invisible, in the Audacity window on the right; finally it seems normal on the TikZiT window on the left.
In addition (I don't know how to take a picture of this, but I'm sure it wouldn't add anything to this question), if I right-click on the Dropbox icon I see a huge font in the white pop-up menu (the same font I see by hovering with the cursor on that icon); when I do the same with Skype (not in the picture) the font i see is the tiny one on the Arc-dark pop-up menu.
I'm new to this do-all-by-yourself-from-scratch world in which ArchLinux and i3 live. And I'd rather not give up after a week or two.
EDIT 1
Here another picture where B are the fonts looking quite ok (I say quite, since I don't even know where they come from), whereas A are the too-tiny fonts. Which are not set by qt5ct.
EDIT 2
I start thinking that maybe is not a conflict between GTK and Qt: as you can see in the following screenshot, I've set similar but definitely not identical themes with qt5ct (left) and lxappearance (right), but pavucontrol (center), which uses GTK, present that tiny unreadable font.
[![enter image description here][4]][4]



GTK_THEMEmay be sufficient.qt5ctand putting[ "$XDG_CURRENT_DESKTOP" = "KDE" ] || [ "$XDG_CURRENT_DESKTOP" = "GNOME" ] || export QT_QPA_PLATFORMTHEME="qt5ct"in~/.xprofile, so now I have another GUI application to set font sizes and stuff, but tiny fonts are still nearly microscopic.