Change icon size in Gnome 3
See this blog post:
- Gnome3 Customization | Change Icon Size
Unfortunately, these system-wide changes are likely to be gone after the next update... To per-user changes, refer to this blog post:
- Per-user Gnome 3 configuration
(It is about Arch Linux, but I think it is applicable)
Install user-theme extension for gnome-shell, then copy themes to your home folders as written, modify them as you like, and choose your new user-modified theme in the gnome-tweak-tool.