I am not sure if there is a setting in Zorin Settings of Core to manage this.
I install qt5ct package. Though with Zorin OS 18, users may need qt6ct, too...
sudo apt install qt5ct qt6ct
In you home .profile add
export QT_QPA_PLATFORMTHEME=qt5ct
Launch qt5ct and then use the Fonts tab to set size.
In Zorin OS... in the past... I have noticed and reported an error that remains unfixed. A file in root /etc will have the incorrect export QT_QPA_PLATFORMTHEME=gtk2
This should be changed to qt5ct. I cannot remember now, off the top of my head which file. But if you search the root /etc directory for the File Content of export QT_QPA_PLATFORMTHEME=, it should come right up. There are only two or three files that would have that in it.
It might be 99qt5ct...