Questions on anal. I really need some info.

So, my SO wants to try anal and I'm a little hesitant. I get the whole if he forces you thing it's bad. He's not forcing me but it's come up a lot and I'm worried about all that it entails. He has a pretty large girth- I can't wrap my whole hand around him- and I'm terrified at how badly it could tear me. Any advice/ info would be extremely helpful. Thank you in advanced.