Deb Wood‘s method of calculating eye size:
An adult is about 7.5-8 heads tall. Let us say, your doll is 27 cm tall and your proportions are 7.5 heads in the body. 27 cm (270 mm), divided by 7.5, equals 3.6 cm (36 mm). The average head is 5 eye widths wide and 7 eye widths tall. Take the number 3.6 cm (36 mm) and divide by 7, you will get 0.514 cm or 5.14 mm. This is the width of the entire eye including the whites. Some people get confused about it and think people are talking about the size of the iris. Therefore, a 27 cm doll will need 5 mm eyes.