So, to apply this here, try running this:If you are using a window manager other than Gnome and have scaled the fonts using Xft.dpi, you must tell GDK to scale the UI as well. This will result in a further increase of the font-size for GDK apps, so you must undo the scaling of the text only.
To scale UI elements by a factor of two:To undo scaling of text:export GDK_SCALE=2
export GDK_DPI_SCALE=0.5
GDK_SCALE=1 /path/to/FreeFileSync
(FreeFileSync_x86_64:4082): Gtk-CRITICAL **: 11:50:04.687: IA__gtk_widget_set_size_request: assertion 'width >= -1' failed
(FreeFileSync_x86_64:4082): Gtk-CRITICAL **: 11:50:04.687: IA__gtk_widget_set_size_request: assertion 'width >= -1' failed
sudo apt-get install libcanberra-gtk-module