void gtk_toolbar_set_icon_size ( GtkToolbar* toolbar, GtkIconSize icon_size )
This function sets the size of stock icons in the toolbar. You can call it both before you add the icons and after they’ve been added. The size you set will override user preferences for the default icon size.
This should only be used for special-purpose toolbars, normal application toolbars should respect the user preferences for the size of icons.
GtkIconSizethat stock icons in the toolbar shall have.