I'm on 20.04. In the settings of dash to dock I've set the icons to small. When I'm in the settings the dock is fine, but when I close the settings it resizes to the default size (big) and stays like that until I have opened the Settings or GNOME Extensions again.
I reinstalled my Ubuntu because I wasn't able to fix this stupid extension and it behaves the same again!
This is how I want it all the time. It's only like this when I'm in the Extensions option:
How it resizes automatically when Settings / Extensions is not open: