Does Size Matter?
My contribution to the "Technique vs. Size" discussion: the shape of the vagina in-situ.
Go here first. Scroll to the 3rd drawing and orient yourself. In particular: the urinary bladder (4)...sitting on top of the uterus (7)...and the vagina (2), to the right [ignore the speculum].
Now, go [WARNING, graphic; pathology specimen] here. This is an abnormal (diseased) specimen, but ignore that. Look at the relationship between the organs; just like in the drawing--the urinary bladder...sitting on top of the uterus...and the vagina, to the right. Notice how the vaginal walls (the top and bottom one) are collapsed; they touch each other. This is what is meant by a "potential space". In the body, the vagina is not tube-shaped. Looking directly at the vulvar end of the vagina, the shape is not a circle (because the vagina is not a tube), but rather ~ H (because the top and the bottom walls are in contact).
[edited for clarity]