How to calculate font height in WPF?

For a FontFamily how do I programatically retrieve/calculate the maximum height range for that font at a particular FontSize?

I need a value to set the height of a textblock that will display the font at the specified FontSize - this has to be carried out programatically.

I need a value that will take into consideration ascenders and descenders, etc.

Update

To clarify, I need the maximum height range for the entire FontFamily, not the height of some sample text in that font. I do not know what the text will be in advance.