Well I have and use 2 basic models of speculuns: the pederson and the graves:
they both come in 3 sizes: small medium and large and the lenghtes are similar comparing the sizes of both models beeing the difference in the widht of the blades: Graves are almost the double of Pederson.
So a large pederson may be nice to a "deep" nuliparous patient and she will feel less strteched than with a medium graves .
Like DR.J58 says "a Medium is capable of opening far enough to dilate almost any vagina sufficiently to visualize the cervix".
Still in some patients a Pederson will expose the cervical os and around but not totally the entire cervix so a graves will be the right choice.
in some cases the examiner may even find an excess of vaginal tissue (in more obese or multyparous patients) that will difficult the acees to a good vizualiation of the cervix. A larger and with greater width speculum like the large graves maybe the solution and the insertion of a codom or latex glove around the blades a good straetegy.
Disposable are mostly (at least in the portuguese market )graves type.