My man has an average sized penis, and I think my pussy is probably looser than would be ideal for his size penis (yes I do have a history of being with a lot of men, and men with large cocks!), however we manage to get by just fine!
I agree with the exercises to tighten up your vag, you can buy some love balls that help with this too, you just place them inside and have to squeeze them to hold them in (they are weighted), you can start off doing that for just ten minutes standing up, then over time work up to longer periods of time and also walking around with them in, this will strenghten and tighten the muscles inside, and also it's a great skill to learn because when you have sex you can use this exercise whilst he's in you to increase his pleasure too.
I've had children so I was taught this by the nurses after I gave birth (they instructed and also gave a pictorial hand out sheet to take home)
Also, try different positions to find which one makes his cock feel deepest for you, personally I like what someone else said about my legs up around his shoulders, and also anal. I also enjoy it if I have a plug or beads in anus whilst he's in my pussy (this seems to tighten my pussy somewhat too).
I have to say that despite his size he knows how to pleasure me with cock and/or fingers and/or toys. I'm not so much in to oral being done on me, but if you like that there's always that too. Although I don't like oral on me normally I must say that he's giving me a really good rimming session a couple of times that even though I didn't want to enjoy it, I really couldn't help but enjoy it.
On the other hand I've been with guys with HUGE cocks before that couldn't satisfy me, so it's all about what he's good at and up for, rather than just the size of his cock! *have also had men with small cocks that couldn't and huge cocks that could satisfy me*
I think the important thing when in a relationship if not being satisfied though is communication! You know what you like so tell him or show him in a sexy way so that he too can learn what it is that satisfies you, because even the worst lay on the planet can learn with the right woman/man who's got the patience and inclination to teach them.