Vaginas are like penises in that they come in a variety of sizes. And, like penises, they also expand and elongate when aroused. An unaroused vagina may only be 3 inches long, but can become considerably longer when aroused.

That being said, everyone is different. Some women like touching their cervix during masturbation/sex, some don't. And there are always exceptions. Also, an 8 inch dildo has to be held, which takes a couple of inches off the toy... so it's around 6ish.

On average, women prefer girth over length (but there are always exceptions).

The vagina does this thing known as the ballooning effect that helps to accommodate insertables. The vagina is a potential space rather than an absolute one. The walls close in when there's nothing inserted. Insertion and arousal change this, however. According to Go Ask Alice, the average of 3-4 inches is for an not aroused vaginal canal.

However, some women do like cervical contact and others don't, which will affect what size they prefer.

As for 8"? Well, I've only ever heard men try to be 8" or more

For my wife, her ability to take length increases with arousal. When we firs start out it can be as little as 5" - when fully aroused she goes to about 7.5" maybe 8". Curve has a lot to do with as well. A straight toy will not go as deeply as one with a nice curve.

It varies. Mine can only handle up to 6 inches.