For what it's worth, I have heard many times from bottoms that they would prefer to be fucked by bigger-than-average cocks... the little ones tend to just stab away at their insides and are uncomfortable.
That having been said, you will want to take plenty of time to get turned on, to relax (especially down there), to get opened up with tongue and fingers (more on that in a minute) and to get lubed up properly. Ask him to go slow. You may find that older guys will have more patience than guys your age, as less experienced tops often go for some kind of land speed record. More experienced tops know that there is plenty of joy to be had in taking one's time.
I've found that one way to get a guy to relax and open up for a big cock is to spend a good amount of time rimming him (licking his asshole and tongue-fucking him). You definitely want to make sure if your guy is gonna rim you that your ass is nice and clean. A douche beforehand isn't a bad idea and there are many types available from over the counter douches to ones that hook up to your shower head. Use lube on both your ass and the douche tip to make insertion easier. (if you want more info on douching, you can visit my site:
http://www.barebackhealth.net/health.html and scroll down the menu to the link on cleaning out)
Finger fucking, starting with one lubed finger up your ass, then two, then three is a great way to get the sphincter to open as long as it is done patiently and in stages. As your hole relaxes more, another finger can be slid in. Three fingers is usually more than adequate. More than that and you're verging on handballing... and you don't want to go there your first time. lol.
Then, use lots of lube, both on your hole and on his cock. Have him gently enter you, and tell him as he is pushing his inches into you when you would like him to pause so you can get accustomed to his size. If you are both patient you can get a very large cock in you on your first try. Save the ass-ramming for a time when you have had more experience
Breathing is an important part of fucking -- especially the penetration part. If it begins to hurt, first tell him, then breathe. Don't hold your breath, That just makes it hurt worse. Deep breathing will also help you relax. And if it just hirts too damn much or if either of you see blood, STOP and have him pull out. Try again another day.
Best of luck to you! Let us know how your first fuck on a big cock goes!