If you HAVE to use magnums, then you're definitely above average. But consider... I don't even come close to fitting into magnums! Hell, I can't roll one down past the halfway mark. Seriously. But I still love fucking a girl's ass. I wouldn't give it up for anything. The secret is properly escalating, from tongue to finger, to fingers, to fingers on both hands, to toys and eventually, slowly, your cock. Ass should always be earned and your girl should practically be begging for it before you actually go through with it. Otherwise she'll be too nervous and her second ring (sphincter) will involuntarily clamp up, which will make it painful for her. Muscle control, pacing and lube are all key to a great assfucking.

^^I've been in that situation before, but the guy always pussies out because of my thickness and how the idiot thinks that I'll hurt his girl

Uhm... try using your tongue. A LOT!

You look pretty average to be honest, both in length & girth.
The majority of girls aren't interested in anal, so you just haven't found the right one yet.

Many girls will only consider trying it with a really thin cock, so get a selection of dildos & gradually work your way through them until she can comfortably take one longer & thicker than your cock.

I've done double anal on a couple girls, with a guy thicker than me!

If all else fails, try a guy

I'd like to help you with your dilemna