How to get the dimension of a JPopupMenu?

Question

I have a TrayIcon with a JPopupMenu attached to it. When I add a JMenuItem to the popup menu, I want to know the dimension of this popup menu. But the dimension is not calculated: getBounds(), getSize() and getVisibleRect() all return zero-by-zero dimensions.

As you can see in the image, the popup menu certainly has a dimension.

Now how can I get the dimension of the popup menu?