photomicrograph and mm2 or cm2

In my professors lab I have been given the task of counting capillary density from rat heart transverse cross-sections. Nobody in the lab knows hows to get to mm2 or cm2 after taking an image. We use image pro plus and an olympus Ix 71 to capture the images @ 20x magnification. Basically when reporting the # of capillaries counted current literature usually expresses it as mm2 or cm2. So after taking my image I am uncertain of the area that I am counting from. How can I figure out how many mm2 or cm2 my counting area is.

Thanks...

have a good one
3Eo