There are two schools of thought regarding the proportions or face dimensions. The first is based on the shape of the head being egg shaped and the other is based on the head being a sphere with the jaw as an extension to it.
Both are effective but fall short of the 'real thing'. That being, what are the dimensions of the portrait you are doing?
Every face has their subtle differences. Barring identical twins and the like no two faces are the same. Even then, over time, each will develop unique differences setting them apart from the other. Consequently, it is important to have good reference material refer-tag so that those differences in the dimensions of the human face can be measured and transposed on to you drawing.
Methods on how to do that will be discusses in the two sections under Set-up refer-tag and Live Sittings. However, as a general rule the face dimensions fall within these parameters.
Facial Dimensions of the Adult Face
Facial Dimensions of a Child's Face
The first thing you need to establish when setting out the face dimensions of your drawing, is to establish the center line of the face from crown to chin. Thereafter the remaining dimensions are set-up perpendicular to that line as per dimensions shown in the diagram above.
The construction lines will act as a map to the structure and dimensions of the face you are doing. Consequently, it must be accurate. The time spent on doing a good set-up will be well rewarded.