Min/max image size parameter - SIFT

How can these 2 param be linked to the actual image size? I mean, even if the overall procedure is scale invariant, these parameters seem very much dependent on whether the image is 100x100px or 1000000x1000000px. Also, what is the unit of measure of these parameters? Pixels? Square pixels? Microns? Would be helpful if in the manuals the unit of measure of the parameters are explicit. Thanks!