The choice of speculum is not only dependent on the size of the vaginal opening, but also on variables such as the length of the vagina, the elasticity of the vaginal walls, the position of the uterus, and what I need to accomplish while up there. As a guide :
The Pederson speculums are designed for women with more narrow vaginas and can be used for ladies who have never had children before.
The Graves are particularly useful in women who have delivered babies vaginally. The curvature allows for better visualization of the cervix.
The pediatric speculum is excellent for patients who have not been sexually active. The population, however, that gets the most benefit from pediatric speculums is ironically the elderly and postmenopausal women.
Sometimes it worth to do a quick vaginal exam , as it can help you to assess the vagina before choosing the right size speculum.