It seems value "Very small icons" (Preferences > Interface > Toolbar) don't work: size "Very small icons" == size "Small icons", and size of toolbar doesn't change. With GTK2 all icon size values variants works correct. Maybe I'm wrong and it's my (local) problem?
Xubuntu 17.10 x64, Geany 1.32, control files (folder "debian") from [here](https://anonscm.debian.org/git/pkg-geany/packages/geany.git).