Your friend found out that your dick really is huge. Gossip from a fling, they spotted you at the urinal, you accidentally sent a dick pic to the wrong person, whatever — it doesn't matter how they found out. But now that they know they just. won't. shut. up. about it. Yeah, it's pretty damn big, but you'd rather be talking about something else, anything else, right now.
The thing to remember about humans is that we're naturally fascinated by the unusual. Tall people garner attention simply for being tall. Women with large breasts get ogled all the time by guys and scorned by some women. And guys with big dicks get all sorts of weird reactions. It's not fair to be treated differently due to a simple fact about your anatomy, but that's unfortunately the world we live in.
There are four categories people tend to fall into when finding out somebody they know has a giant dick: whatever, fascination, sexualization, and jealousy. "Whatever" is an easy one to handle — it's just another fact about you, but it doesn't change who you are and how they treat you. It's not like they found out you murder puppies on the weekend, you just happen to have a large penis. Maybe they'll joke about it every now and then, but they also joke about that time you spilled spaghetti all over your white shirt.
The curious friend with all the questions
It's worth remembering that huge dicks are exceptionally rare. So upon finding out that you've got a big one, sometimes people can become really inquisitive about it. They want to know what sex with an enormous cock is like, were you always so big, how many women it gets you, and so on.
The best approach to handling these questions is just to answer them in a matter-of-fact manner. Be honest and straightforward, work in the full picture of the pros and cons you've experienced, and don't be afraid to tell them when you're not comfortable with the questioning.
Curiosity about the unusual is natural. And so is setting boundaries.
The friend who is suddenly really into you
Tall guys and big-breasted women can relate: the unusual attribute becomes who you are in the eyes of some. They can't hide those attributes easily (just be less tall, dude), unlike a big dick which can come as a huge surprise to some. And then once they find out everything changes about your interactions.
Suddenly it's all about your big dick. Not you, just the dick. Every conversation is about it, you're getting constant glances or outright stares at your crotch. Maybe they're even getting handsy in a way you're not comfortable with. This is the time to stand up for yourself, but it takes finesse that's dependent on the person. In any instance, blowing up at them for treating you like a piece of meat generally isn't the best approach. Be firm, but understanding, in telling them that you'd rather they treat you like they did before finding out. You're a fully formed human being, not just a penis.
Or, maybe if you're into that friend, you can use the opportunity to get laid. Just be aware that such a move can fundamentally change your relationship, and not necessarily in a good way. Up to you whether you want to take that risk.
The friend that turns jealous and bitter
This is the worst reaction, and it typically comes from average guys that are massively insecure about their dick size from years of porn messing with their brains. They become shockingly jealous and hostile, treating you like competition to be vanquished or bringing up your size to be dismissive about anything else you've done. Lines like "She's only with you because of your huge dick" and such. It's highly negative sexualization, diminishing you and everything you've done to just a penis.
This is the hardest reaction to overcome, as you're being treated poorly simply because of what hangs between your legs. Odds are good this one will take a long discussion and requires lots of calm on your part to keep your friend from flying off the hook again. It can be hard to play therapist to somebody you care about, but in all reality their sudden displeasure with you is rooted in something else that happened in their own past — an ex telling them they were small, undeserved juvenile mocking at the pool, a horribly skewed perception of penis size due to porn, etc.
So be honest with them. Tell them it's just a penis, that it doesn't define you, who you are, or your friendship with them. Try and work through why it's got them so upset and then work to move past it. Remind him of why you're friends in the first place, and that your dick doesn't make you the competition.