I'm not aware of anything specifically for that purpose. Have you tried the measure tool? You might also find Senocular's Transform panel useful. http://www.senocular.com/fireworks/extensions/?entry=572
Thank you Linda, the transform tool couldn't calculate (I couldn't make it do it), but senocular.com has some other good stuff, "Used fonts" for instance.
But no recent ones, all seems to be a few years old.