Thunar seems to be a GTK 3 based application in Garuda Hyprland. Add the GTK CSS configuration to ~/.config/gtk-3.0/gtk.css file, creating that file if it does not exist.
I set a font size of 16 pixels that works well for both my 5k2k external display and 1920x1080 HD laptop display.
/* Set font size for thunar file manager */
.thunar { font-size: 16pt }
If the system has two monitors of different resolutions, a monitor definition for each display should be added to .config/hypr/hyprland.config or a monitors.conf which is sourced by hyprland.conf
Without specific monitor definitions it could be challenging to find a gkt.css font-size that would work for multiple displays.