In researching marital aids for a new story, I ran across this ad copy for "The Perfect Marital Aid Prosthetic Penis":
The Perfect Marital Aid for most people is realistically sized
Overall length is 6" with an outer diameter of 1.675" and 1.125" inner diameter
(It's on Amazon here.)
No one wants to be a killjoy, so feel free to continue with whatever cock size suits your story, but the marketers have spoken.