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.
This is one of those little portrait painter secrets that hides in plain sight in just about every "old" portrait you will find hanging in any museum.
Below are some more extreme examples so you can easily see and understand this. Velazquez made King Philip IV of Spain look very "kingly, noble and intelligent" by merely adjusting the proportions of his body to be larger and "shrinking the size" of his head.
The painter also chose a perspective point slightly above our eye level so that in order to view the King's face, we must "look up" to him. This painting is very large ( 82 1/8" x 43 3/8" - and with the heavy frame, it looks larger still)- yet the head is less than life size!
The king's only decoration is a golden chain. Most royal portraits of this era emphasized lavish displays of clothing and props but Philip IV is soberly represented in black. Because of the artist's skill in employing this "head size" trick, the importance of this great and powerful ruler is self-evident.
We can easily translate "understating the head size" into contemporary portraiture. I employ this technique when I want the personality to appear as an intelligent and worthy leader. (i.e., when painting a president, CEO, etc).
However, I would suggest that this "painting trick" be very subtle and not appear as extreme as these examples (although Velazquez's example of this above illustrates the concept so well).
When the subject is painted with a large head (as in the pictures below), even though it looks perfectly normal in reality, this "large-head" feature tends to make the sitter look a "wee bit stupid" in a painted portrait.
John Singleton Copley probably painted the actual sitter's head size accurately, but it doesn't really flatter or make The Portrait of Eleazer Tyng (below) look very bright.
It was tough to find any Old Master Painter who painted heads too large so I offer my apologies to John Singleton Copley for picking on him for these examples.
Below is another example of a head (The Portrait of Hon John Erving) that is painted at an unflattering size.
And yet another "big head,"- herewith The Portrait of Exekiel Goldthwait. If anything, the clothing, wig and background devices may suggest his "importance."
I define "head size" to be the measurements from under the chin to the top of the skull (not the top of the hair).
Most adult head sizes average 9 inches...give or take....
I prefer to not paint any head larger than 6 1/5 inches (3/4 size) no matter how large the canvas size.
The larger the canvas, the more important the clothing, props and "background" become.
Below is a large painting - still with less than life size heads but lots of background.
Above is another copley "Mr. and Mrs Ralph Isard," and this canvas size is 68 3/4" x 88." The viewer is so "taken with" all the interesting things happening in this painting and Copley's painterly brush that we hardly notice those shrunken heads on life (or larger-than-life) size bodies.
* Note, there is a little matter of catchlights in the eyes - where you put them will give a sitter a decidedly "dull" look or make them appear bright-eyed and bushy-tailed.
But that is for a later post, stay tuned.