Geany can be built for GTK2 or GTK3, just not both at once in the one executable, although if built with different prefixes you could have both available.
The GTK3 icon size is taken from the system theme, so blame whoever set the theme you use. Oh wait, that would be you (even if by default :-) On my Linux Mint there is a cheery dialog that lets me set the icon theme. I expect Slackware has something similar.
You can also set Geany specific overrides in the geany.css in your config directory.