How do I measure the size of a TextBlock in WPF before it is rendered?

The call to UIElement.Measure(Size), takes as a parameter Size. The second call UIElement.DesiredSize returns whatever Size you passed into the Measure method.

I think this is the case because UIElement (TextBlock in this case) is NOT a child of any control (yet) and therefore DesiredSize has no reason to be anything different.

I think it should be sufficient to call the UIElement.Measure(Size) method and subsequently check the UIElement.DesiredSize property. For more information, check the provided MSDN links.