Hi Blackbird,
The tool from Iconico can be set to any units.. The actual size would have to come from a known reference on the screen...

I use Iconico screen calipers all the time in several graphic apps. Works with any app because it sits on top of your screen.

It measures pixels, but you can calibrate it to any unit of measurement. If you want 88 pixels to read out as 12.5 feet or 0.014 inches, no problem.

It rotates, making it nice for measuring the length of an object that is on a angle.

Ed

Iconico also makes a protractor and compass:

http://www.iconico.com/