@Byebye I agree, the high is important for this, and I believe someone once told me that in the book, the height also the focus too.
Since I saw picture of Nadech and Mew in TRKJ, their heights difference is perfect for this, and also I have no problem watching Nadech taking Willy's roles. I know he's a newbie and not so versatile with his roles yet, but if he can take this roles and proof himself he can act, he'll be great.
With veteran actors that I sees fit for this roles would be Andrew and Tik, but Tik is kind of short, and Andrew need to be better fit than he is now.