To answer your question appropriately I need to ask why you want to know the dimensions and measurements? Are you looking to make your own visor, or perhaps checking a visor to see if it suits your car?
The outer visor brackets have either two or three mounting bolts that attach to the top of the seam so the width is the distance between the seams. The number and exact position of the holes will depend on the type of visor.
Here's the BMC factory accessory visor that was fitted to my car.
Attachment:
Sunvisor b.jpg
Attachment:
Sun Visor Holes Front Seam.jpg