From what I know length and girth are both important maybe leaning toward girth more. The anatomy of the clitoris now much better understood shows us that it goes all over the place inside the vagina. On the top as the G-Spot, on the sides in contact with the vestibular bulbs. The perineum may also be part of the same nerve network although not physically connected to the clitoris proper.
see:
More girth means more friction on the vestibular blubs. All the scientific jargon talks about most nerve endings being in the first 2 inches of the vaginal opening. Another thing girth does, and you can see it if you watch videos and the guy is wide enough, is that it pulls the external clitoris and hood on every in and out stroke.
As for length, there are spots to hit (g-spot, a-spot and the area under the cervix, between the cervix and the anus) and I think some women (many?) like the feeling of being filled. If you see this thread by luvdadus (Collected wisdom from the vets and good gainers) where he says:
"When I got to a little over 7" bpel. I noticed that the wife seemed much more approachable for sex overall and initiated much more of the time. I postulated that I must be hitting areas that I hadn’t been able to before and the orgasms were more intense and pleasurable for her hence the increased interest. After reading up a bit I decided that I was getting to the cul de sac area at times since I had noticed that the cervix would slide over the head of my dick at times - when it did orgasms ensued shortly therafter for wifey. Over time I noted an increase in the frequency of sex with my wife from about 10-11 per 26 day cycle to 13 or 14. It was when I got to 7.5 bpel that I learned that the wife was haveing some "discomfort" from time to time during sex. The "discomfort" however never resulted in her reducing her interest, in fact if anything her interest continued to steadily increase. None-the-less I ended up spending 3 or 4 months doing girth only routines or on a PE vacation till she seemed to have accomadated to the new length. I did this because I felt that PE should enhance pleasure for her, not just be an ego booster for me. I definitely got the benefits of better erections, I also got a bigger dick, a more sastisfied spouse and a good deal more sex in the bargain."
Reference on the A-Spot:
"The A-Spot, AFE-zone or Anterior Fornix Erogenous Zone. Also referred to as the Epicentre, this is a patch of sensitive tissue at the inner end of the vaginal tube between the cervix and the bladder, described technically as the ‘female degenerated prostate’. (In other words, it is the female equivalent of the male prostate, just as the clitoris is the female equivalent of the male penis.) Direct stimulation of this spot can produce violent orgasmic contractions. Unlike the clitoris, it is not supposed to suffer from post-orgasmic over-sensitivity. "
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Starting: 5.5" BPEL, 5.0" MSEG (Jan 2012). Current (4/9/2012) 5.75" BPEL
Immediate Goal: 6.5" x 5.5", Continuing on to: 7.5" x 6.0" and eventually would like to reach 8.5" x 7"