Even when they are 1", little koksters show their head.
As for how to promote growth of the kok, Clean water, good diet, and good sized aquarium should do. Rest is all about the genes.
Now the things that you should avoid because it will shrink their head:
1. Put it in the community tank
2. Do not put the mirror too long
3. Do not put with other male divided up
Hope this helps,
JP