I copied a table from a manufacturer of surgical steel Vaginal Specula. (Department of Useless Information)
My point being that a small difference in size can seem to make a very large impression on a patient. The vast majority of women can best be served by the medium size.
You don't measure how far it will open. Simply stop expanding the device when you can visualize the cervix and before she kicks you in the head. Most of them are available with larger yoke opening, the smallest for diagnostic work and the largest for surgical procedures...or as sometimes seen in some pornography.
Vaginal Speculum Approximate Measurements - 30 mm Width 1 ΒΌβ; 40 mm Width 1 Β½β; 70 mm Width 2 ΒΎβ
Graves β most common
Small
22 mm x 87 mm ( 7/8β x 3 Β½β)
Medium
33 mm x 113 mm (1 ΒΌββ x 4β)
Large & larger
35 mm x 125 mm (1 3/8β x 5β)
35 mm x 150 mm (1 3/8β x 6β)
Pederson - Adolescent/Petite woman
Small
16 mm x 84 mm (1/2β x 3 ΒΌβ)
Medium
22 mm x 114 mm (3/4β x 4β)
Large
23 mm x 121 mm ( 7/8β x 4 ΒΎβ)
26 mm x 150 mm ( 1β x 6β)