GtkSettings.
(See gtkIconSizeLookupForSettings.)
Normally size would be
GTK_ICON_SIZE_MENU, GTK_ICON_SIZE_BUTTON, etc. This function
isn't normally needed, gtkWidgetRenderIcon is the usual
way to get an icon for rendering, then just look at the size of
the rendered pixbuf. The rendered pixbuf may not even correspond to
the width/height returned by gtkIconSizeLookup, because themes
are free to render the pixbuf however they like, including changing
the usual size.gtkIconSizeLookup(size)sizeTRUE if size was a valid sizewidthheight