Great question. Like you said, foreplay will never be a bad thing--during the stages of arousal, the cervix pulls up and causes the vagina itself to lengthen; this alone helps (and it helps all women, not just those with larger men!). Otherwise, it may be better for you to veer away from certain positions. If you're on top, you have more control of how deep he's going; doggy-style is notorious for deep-thrusting, so that one's going to hurt a bit more with a larger partner.

Since the vagina is made of muscle that stretches (think of childbirth), technically it can accommodate large sizes. Penetration, however, may be very uncomfortable or even painful with deep thrusting because of the way it pushes on other structures (cervix or even ovaries at certain angles). As Anna mentioned, you need to be aware of what positions increase the likelihood that you would feel this way so that you can either stay away from those with a man who is large, or use your legs/hips to control the depth of penetration. You need to have good communication with your partner to let him know what pleases you and what hurts you. If you find that you have a great deal of pain with deep penetration, ask your ObGyn to evaluate you for endometriosis at your next visit since this is one of the most common symptoms.