The gauge depends on the type of yarn you use. You could also use thread with a steel hook for smaller skulls.
If you use a thinner yarn, you may want to use a different sized hook. A bigger skull would call for a thicker yarn and a bigger hook, etc.
I used a worsted weight yarn with a size H hook. it came out to be about 3 3/4” from the top down, and 4” across the eyes.