How do you estimate the size of a cell when your using a microscope?

put ruller under the MS and mesure the diameter rember 1 milliter equals 1000 micrometers so if the diameter which im guess you no what that is say its 2.7 millimeter times that by thousand and it become 2700 microns now you count how many cells fit in that dimater to get the approximate lenght so if 6 fit divide that by your 2700 which is approximate length of 470 micrometers for each cell you don't need the presice because that's impossible for a student