In my experience, I had a harder time with length than girth, and as several have pointed out, the answers are all subjective. Different women have vaginas of different depths and some aren't comfortable with more than 6" in length while other women here have stated that they prefer 9"-10" long cocks. Our vaginas are as different as cocks are. Some women can handle girth much more easily than others can, too, although like MB said, I will stretch out, which only makes girth a problem the first one or two times we have sex.
My own experience is I can handle more girth when I've been having regular sex or if I play with insertion toys regularly to keep me stretched out. TheBF says that I feel tighter to him again after only two days, which is interesting to me because that means that he notices it before I do, but he's never actually commented on how much tighter I am until we haven't had sex and there's been no toy play for me for two weeks, and then I can't have sex for very long and I'm usually still sore the next day even if he's gentle. I might be sore for 3 days if he's not gentle, which is sometimes my fault, too. When we've been apart, there have been a few times when I was overly enthusiastic about making his homecoming really good, memorable and fun and to show him how much I missed him, sometimes at the expense of my poor vagina, and that's when I'm sitting on frozen peas the next day. There have been a few times when I've told him I want him to fuck me harder and that I would be okay, and he hasn't wanted to and been gentle and careful with me, only to discover the next day that he was right and I was wrong because I was still sore even though he had been gentle, so I would have felt torn up if he had gotten rougher with me. I think I tend to forget about the last time things went wrong when I'm in the moment, but he remembers.
One time when we were apart for a month and I had not been using insertion toys to masturbate with (this was before my toy collection was large like it is now) I could only have sex with him for about 15 minutes before apologizing that I really just couldn't keep going, it was just too uncomfortable. It was disappointing, but if I remember correctly, the next day I wasn't too sore to have sex and having sex with him didn't make me a lot sorer, and the day after that he fit perfectly again, so stopping after 15 minutes was the right thing to do, even though I felt awful and lame and I felt like I ruined his homecoming. So my vagina adapts quickly to girth as long as it's done right. It's not an immediate adjustment, but it's still fast. Now I regularly use insertion toys when he's coming back home again, so that we don't have those problems again and we don't as long as I properly prepare for sex with him.
For me, length was totally different than girth. Sure, the more aroused I am, the more cock will fit inside me, but there seemed to be an absolute limit to how much length I could take before I got pregnant, and that absolute limit was always shorter than TheBF's cock. No progress ever seemed to be made on that, no matter how many toys I used, how long we had foreplay, or how much we fucked. He just didn't fit all the way inside me.
Then I got pregnant and early on in my pregnancy I read an article on pregnancy sex that said that one of the hormones responsible for transforming my insides into a baby incubator would affect my vagina so that it would be roomier and deeper than before. We were really excited about that, but month after month passed and nothing had changed. Then around the 5th month of pregnancy, I suppose my body had enough relaxin in it for long enough and my organs had been
pushed around enough that ligaments and tissue loosened up and suddenly his cock fit, just like the article in the pregnancy magazine said it would happen. I talked to a few gynecologists about it and I was told that it might not last for my entire pregnancy, and that my vagina may be different after my pregnancy than before, but they could make me no promises about that. It's possible that his length will be a problem again once I've recovered from the birth. No one knows what will happen. I am sad to report that he's been bottoming out a lot over the past month, so our period of free for all fun lasted for about 10 weeks. The last time he bottomed out it was extremely painful, so we're back to being cautious and careful about that again, wihch isn't nearly as much fun. I've got my fingers crossed that things will be different after I deliver.
