The first thing to do is to decide what size your doll will be, then determine what size the head will be.
This tutorial will help you with these calculations.
"Dolls seem to vary from 6 to 18 inches, so I need to figure out a scale for my dolls. I'm going to use 10." That seems fairly standard, I want the big enough to be able to add a lot of detail, but not huge. First, I plan to make three. A mother and her two daughters. So the mother will be 10." The older, tall daughter will be 11" and the younger, buxom daughter will be 9." I'm starting with the mother.
"You can't really say that a female doll will be 6 inches or 12 inches tall, because every doll can be a different size. So instead, we will measure in "head lengths".