In GTK, how do I get the actual size of a widget on screen?

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.