GUIText width height

I want to center a GUIText below a GUITexture.
Therefore I need to know the width / height of a GUIText.

How can I get that?
I couldnt find anything in the GUIText class…

There is the method

GetScreenRect() for GUITexts

but it looks like this is set to the rectangle of the initial text.

So when I change the text the Screen Rectangle does not seem to change…