Just relax. I cried the first few times I had sex. It's OK to cry.
I try to remind myself that my body is giving someone else pleasure, and that is worth any pain.
It's your body and your own choice, but you will get used to it no matter how big. I have been hurt plenty of times, but I don't regret any of them.
When my boyfriend wanted to fist me I just cried with my face in a pillow but let him have his fun. I try to think of it as accomplishments.
I'd never tell him this, but I secretly wish he'd do such extreme things more often so I could try to get used to it. I know that he had a lot of fun, and that in itself is a huge turn on.