Sketching is a work in progress. You'll make adjustments to the sketch when comparing and A sewing gauge is a good tool for measuring.
1. Find a Comparison Measurement
In this example, the distance from the eyes to the base of the nose with be the unit of measurement.
2. Use the Measurement
This unit of measurement happens to be equal to the distance from the base of the nose to the bottom of the lower lip.
Excerpt taken from, "Drawing Portraits for the Absolute Beginner."