@Azazaza:
I've had three fuckbuddies.
One was a bit below average. I even asked the dreaded "Are you in?" question once.
The next was a bit above average with a good mushroom tip. It hurt going in, and when it popped through, then it didn't hurt at all, but made me feel full inside and massaged me nicely.
The last was really big and attached to a heavy guy who really liked to pound. It hurt me so that I was involuntarily moaning all while being fucked and my ass burned and tingled for a day afterward.
I only fantasize about the last guy now.
I'm the same way. Total maso-slut nowadays.
To answer the OP's question, the primary factor is how relaxed you are. It is entirely possible to work your way up to any cock size or dildo size with preparation and care and have absolutely no pain involved. Rimming/reaming is very helpful in this regard, since the anal ring has pleasure sensitive nerves, and a tongue is basically incapable of causing pain. The pleasure helps you to relax, so that you can easily take small to average sized dicks without pain. Larger cocks will require more prep, either with fingers or toys, to get you used to having an object inserted and stretch you out a little ahead of time.
For most guys, I personally don't need any prep because I love receiving and I can relax myself back there without effort. Those muscles are under your control, but you'll probably need some experience receiving before you learn how to use them that way, since they are mostly autonomically controlled.
On the other hand, if you're really nervous and can't relax, and your partner refuses to prep you or doesn't know how, it'll probably hurt. Always remember; 2 virgins + A bed = A mistake.
Oh, random sidenote; someone else said condoms help, but for me it's always been the opposite; they pull at my anal ring and make what should feel good very uncomfortable.