I think I am going to let my friend's boyfriend know that her ex had a huge cock.
What is the point in telling your friend's boyfriend something like this.
I want them to worry about my friendship.
It's very wrong to try to induce anxiety in someone by talking about the physical qualities of their ex's that they may not measure up to. Doing so will cause that person to worry about something that they cannot change and will make them question their relationship and ability to keep their partner happy. Don't bring up details about past exes with a person who does not want to know about them and avoid spreading rumors that will damage a relationship.
My intentions are to hurt him and make my friend loose her mind.
It is just kind of passive aggressive to make a comment like this just to have the boyfriend become upset. You are better off leaving them alone so they can be happy together without any outside stressors.