How do I calculate the illuminated spot on specimen in optical microscope?

Hi,

When we use an optical microscope, how do we roughly calculate the illuminated spot size on the sample ? Also what is the light power on that spot?

And what if I change to fluorescence microscope(epi illumination)? Cause the illuminating light go through the rear side of objective to the surface of specimen, also how do we calculate the spot size?

Thanks!