I used to be friends with someone who was very big. I didn't believe him for years until he showed me one day. We both liked men, but we weren't into each other sexually. Just friends. I don't remember what lead up to that moment but I think he'd bought some pants or shorts that showed him off a little to well, and it ignited that playful argument we'd always had. Yeah, he was big. AFAIK it's always been a problem in his relationships. People like the idea of a big dick, but it's not comfortable long term. I know he briefly dated someone we both knew, whom I wasn't well acquainted with. I do remember said person talking about the sex and how it was difficult for him to take it but didn't comment on whether it was enjoyable or not. I didn't press the matter.
IMO I'd say 7" is where things begin to get difficult. I say begin because it could be a thinner dick or it could be a baseball bat. Although you can say this with just about any penis size down to a certain point. I've never really measured myself but I think up to 7" would be fine for most people. 6.5" being the Goldilocks size.
I like to think there's a golden ratio between body layout, dick size, and dick prettiness that makes a dick attractive enough to cause your mind to race compared to a "Oh another nude dude with his dingus out, yay" level of enthusiasm. Or in plainer words, there's dicks attached to dudes out there that make you want to go Alaskan bear on wild salmon out of a stream.