Help to find the Image magnification factor and pixel size ?

Hi all, I am imaging some nano meter particles in a flow. I would like to find the diameter of these particles. I used regionprops to get the equidiameter. Now, all the values are in pixels how do i convert to required scale(unit length). My sensor size is 6.45um x 6.45um . I also use a magnification lens. I know that the magnification factor has to be used. How can i calculate M ?

Pls help! Thank you