AFAICT with the exception of the save all, close all and fallen over bookcase (build, I think its supposed to be a brick?) all the icons are standard ones provided by the GTK theme. The three Geany ones are provided as SVGs so are scalable. GTK only provides the application with settings for sizes default, small, medium and large, and the actual sizes those represent are selected by the GTK theme I believe, not by Geany directly.