https://developer.gnome.org/gi/stable/gi-GIFieldInfo.html#g-field-info-get-size
I asked already in https://mail.gnome.org/archives/gtk-list/2017-February/msg00014.html
some years ago. Not too important, as we generally ignore fields in high level bindings. But for some structs like GtkTextAttributes with bitfields it may be nice to get a hint that bitfields are involved, so user level allocations should be really avoided.
I just looked at the source code of that function at github – it is not obvious that it always returns zero. So it is not fully impossible that I am using it wrong.