One way to do this would be to look at a pair of facial landmark points on either side of the face and measure their distance between their coordinates. You can read individual face points with an instruction called QueryPoint.
The values of particular landmark points are on a face chart.
Looking at the chart, you could use the furthest-out points of 53 and 69 at the left and right ears to make your face width calculation.
It is possible that you may also need to use another instruction called ProjectCameraToDepth to convert the image coordinates of the face that the camera is seeing into real-world XYZ coordinates.