I'm about 7 inches when fully turned on and there've been 2 girls I have to be careful with. 1 was petite and of course when you're screwing her and she's getting off on it she'd always want more and for you to ram in hard all you've got - which she (and I) then quickly realised wasn't a good idea at all - we only did this a couple times before I learned to hold back some even if she's telling you she wants it all. So no going balls deep there. There again, like I said, she was a short girl, so maybe that had something to do with the size of her vagina.
The other girl is larger, and she can pretty much take it all, almost all the time, provided we work up to it properly - I start off using about half my length, and then a bit more and a bit more until it's all in. There have a few occasions though when I've either used too much or been too hasty with it (once missionary, twice from behind) when it's been uncomfortable/painful for her too. My guess is that different positions affect how much 'effective length' you've got inside her; all it means is I have to be more careful when I'm taking her from behind. I would say that you can't predict how you will be with any girl - just take it easy to begin with and if anything goes less than great, learn from it and remember it next time - it's worked well for us anyway.
I'm much happier now than with the previous one - being able to push and pump away without having to worry about how much you're putting in (and worrying about hurting her or making her uncomfortable, which I really hate) is wonderful - and 99.9% of the time now, that's just how it is together