I remember that I read in a medical research that the length of the vagina's tube is ~7-7.5 cm. If you are around this number, you should be fine.
How about the tightness of the vagina? you guys never concerned about it? I mean sometimes I go with someone where I can stick my whole hand inside, and really don't feel tightness. When this happen I just work it with HJ. Sometime, they excuse me, and go put something like alcohol inside their vagina to make it tight, and it works!
Also, the ladies can work that out by doing exercises for example to make it tight. For men, it's almost impossible to change it.