Head size and intellect?
 

Although I paint from photographic reference material, I don't feel any need to be confined by that "reality" - most especially when it comes to an anatomical proportion like head size.

When the subject has a large head, even though it looks "perfectly normal" in reality, this feature tends to make someone look a "wee bit stupid" in a portrait painting.

Below is an example of a portrait by John Singleton Copley in which the sitter's head size is probably accurate, but it doesn't really come across well...

And here is a portrait of King Philip IV by Velazquez. You can see that this subject has a verrrry small head in proportion to his body.

Philip, because of his head size (certainly not his physical beauty), comes across as an intelligent and worthy leader...

If anything I will err on the side of understatment of the head size also. It provides the statuesque Giacometti/ Charles DeGaulle look of prominence and authority. I think Phil #4 however, looks like a pinhead.

I chose the most extreme example of a small head that I could find in order to make the point.:sunnysmil