First I looked at the get_size_request method. The docs there end with:
To get the size a widget will actually use, call the
size_request()instead of this method.
I look at size_request(), and it ends with
Also remember that the size request is not necessarily the size a widget will actually be allocated.
So, is there any function in GTK to get what size a widget actually is? This is a widget that is on-screen and actually being displayed, so GTK definitely has this information somewhere.