Eyes are usually sized according to the measurement of their widest point. To measure an eye, slide open the measuring jaw of a caliper tool. Place an eye in between the jaws and slide them together trapping the eye at its widest point between the jaws. Read the size show non the tool. Alternatively, if you do not have a caliper tool, use a soft measuring tape and measure the distance between the inner and outer eye corners in millimetres. This works just as well.
A handy guide for all the Berenguer doll eye sizes can be found here: www.rebornsupplies.com/eye_size_chart