PGF and TikZ -- Graphic systems for TeX

Dear all,

For some reason, I need the width of a tikz node in
centimetres. The node has a label, and ideally I'd like to have a
macro that returns the width as a function of the label. I only
intend to use the macro in the tikzpicture that draws the node, and
after the drawing of the node. Is there an easy way to implement such
a macro?

Any help/pointers/suggestions will be greatly appreciated.

Regards,

Marc van Dongen