During the night, they started getting into talking about exs and dick size. I'm thinking to myself Yall know I can hear you, but instead I just try to ignore it. But the girl I was dating made a remark about how one of her ex's had a big perfect dick. I'm thinking wow really? I'm rolling with an average 6" which you've seen and want to talk about another dude's big dick sitting next to me? So I make up an excuse and take off. Next day I text her we should see other people. She texted back with she didn't understand but ok and asked if we could still hang out sometime. I didn't respond and just put it behind me. The following week I met a new girl who seems really cool and am still dating.

I think you handled that very well. Personally, I wouldn't knowingly want to date someone who prefers bigger than me either. Not to mention her showing how willing she is to share intimate details like that is highly disrespectful, and doesn't really show her to be all that trustworthy in a relationship, when it comes to what you can talk to them about.

she starts up with how I'm "an insecure idiot for dumping her over a preference."

Women are entitled to prefer bigger than average. Men who aren't bigger than average are entitled not to date those women, if they don't want to. You've already moved on, so I'll say you're doing right for you.

As for your friend, sounds like she's projecting a bit and can't comprehend the fact that men really do end relationships over this. Perhaps she can relate to her classmate friend more than you, because she's been in her.shoes? Or perhaps she's catching shit from her classmate friend for you dumping her. Regardless, if your friend is about to throw away a longterm friendship over you exercising your right to choose who you'll be with, then she isn't really much of a friend.

Let her calm her shit and come to you. Focus on your new lady, your life, and keep living. You did nothing wrong.