Should I tell my boyfriend the real reason I'm leaving him?

My boyfriend of 7 months is a great guy and I really like him, but he has a pretty small penis and I just can't see myself getting serious with someone who doesn't offer everything I need in a man.

Penis size has only come up a few times and I've told him he was big enough, but he really isn't and I'm not sure how to tell him.

He made me promise I would be truthful with him and I really want to be but I don't want to hurt him either.

He's only about 5.5" in case anyone is wondering.

Need advice Reddit, what should i do?