From what I can see in the picture, the cell appears that it would take up half of the microscopes view if it were to be pushed to the very edge on one side.
That would be the radius. The radius is half of the diameter, and half of 0.8 mm would be 0.4 mm. The length of the cell is about 0.4 mm.
Now for the width. Since the diameter of the FoV (field of view) is 0.8, we can see that the cell doesn't reach both sides of the circle it is in. It also doesn't appear to take up half of it. So, to get the aprox. width of the cell, we're going to half the radius.
Half of 0.4 mm is 0.2 mm, so the width of your cell is about 0.2 mm.