As uncut (Ya man, FANTASTIC meat BTW) says, go slllow. That's the key to it all. You might even want to work your way up to a huge dick. Don't just go for the biggest right off the bat. You may even want to try a dildo or two...get your ass used to be stretched open and invaded deeply.
When I was in my 20's I wasn't much into being fucked, made a few attempts, the guys weren't large by any means, but it just seemed uncomfortable. Then I met a guy who I saw for a while, he was only about 6" long, but a solid 6" around also. Maybe it's because I love thick cock, I don't know but I was eager for him to fuck me. And well ever since then, I've truly LOVED to take it up the ass. I've had 10x6 (and a bit) up there and it was no problem...he liked to REALLY POUND too.
There is no way I could have possibly handled that 10x6 if I hadn't taken the time to open up my ass a bit with lesser mortals.
The go slow thing definitely applies at the moment he is entering you too. Make SURE you have someone that will listen to you and STOP if you tell them to stop. If they shove in to fast, well you will be in a lot of pain, it will ruin the entire fuck AND your ass will probably hurt for a few days afterwards. That's never any fun.