Normally if I'm buying eyes for my 60cm boys I get 18mm and they look fine. Souldoll eyes however have larger irises and therefore look a little different. Donovan's eyes are larger than the average Yder's but not by a lot, so you still get the point.
18mm:

16mm:

Keep this in mind if you buy souldoll eyes! They are gorgeous but personally I'm sticking with 16mm and when I finally get around to buying Key's eyes (the purple ones) in 16mm I'll be selling the 18mm ones off.
Oh and feel free to laugh at Donovan in the first pic, either for the eyes, the "OMGWTF?!" look in general or the blond hair. He makes a horrible blond. xd