Currently to get the size of a widget you need to do this:

  Size get _size {
    assert(mounted);
    RenderBox box = context.findRenderObject();
    assert(box != null);
    assert(box.hasSize);
    return box.size;
  }

It'd be nice to have this as a built in api.

((I'm copying this code all over the place.))

Similarly getting the size of the widget via GlobalKey would be good too. Maybe something can be added to GlobalKey?