Sounds fishy..
but here's the advice...

Human females can, if willing, accommodate a much larger and longer penis in the ass than in their vagina... but it takes some time to 'train' for that, and a woman who wants it that badly.


And some prep work prior is also a good idea... which can take the spontaneity out.


Short of that... wrap some thick leather or rubber cockrings around the base of the shaft-- stack up a bunch to cover half the length--- this will enable you to at least have the feeling of slamming it home without damaging a woman internally... as the rings will act as a padded bumper to stop further penetration.


beyond that... if you really have so massive a wang... you could be a demi-god at the local bathhouse.

And they would not make you feel like they were doing YOU a favor by letting you have at them, either.