I have the following code:
(...)
        gtk_status_icon_set_visible (netapplet->tray_icon, TRUE);
        netapplet_set_icon (DISCONNECT_ICON);
(...)

Being netapplet_set_icon:
(...)
        panel_h = gtk_status_icon_get_size (netapplet->tray_icon);

        if (panel_h < 30)
                icon_size = 16;
        else
                icon_size = 24;
        g_print ("icon_name: %s; panel_h: %d; icon_size: %d\n\n", icon_name, panel_h, icon_size);

        pixbuf = gtk_icon_theme_load_icon (gtk_icon_theme_get_default (),
                                           icon_name, icon_size, 0, NULL);

        gtk_status_icon_set_from_pixbuf (netapplet->tray_icon, pixbuf);
        g_object_unref (pixbuf);
(...)
This will g_print():
icon_name: something; panel_h: 200; icon_size: 24
after the first call to netapplet_set_icon I would see:
icon_name: something; panel_h: 22; icon_size: 16

I'm understanding that GtkStatusIcon isn't aware of the notification area size until it's asked to draw something (in this case a pixbuf).
I *think* that this shouldn't be like this, I don't know what will be the size of the icon until I set an icon in the GtkStatusIcon (ironic).

GtkStatusIcon should tell me the notification area size so I can set my icon size correctly (and not just guess it).

Note: I know that I should use a GtkIconFactory and make my icon go into stock so I can do gtk_status_icon_set_from_stock (), but I noticed this bug and I don't think that a bad practice (of the programmer) should be excuse for an unexpected behavior (of gtk+).

Note2: If I'm saying nonsense stuff I will understand if you mark as NOTABUG :). Be kind, I'm not much of a GTK+ master.