On my screen 50um = 1.625"
the dot measures .0937 at most but I read it more like .0625
That puts the supposed red blood cell at 2.88 or more likely 1.89.
If we allow for the reduced size caused by pixel artifacts it is probably about 1 micron.
Simply counting pixels and using 3x 3 pixels for your red blood cell it is 2.88 um. The line is 52 pixels long.
(3/52) X 50 = 2.88
Based on the color of the side pixels, the dot is really about 1.5 pixels wide.
What I really want to know is how you get that 9 pixels is a red blood cell. I can find similar artifacts in every graphic online. A 9 x 9 pixel with the center darker is easy to find.