Lucky for alfie, penis size isn't heredidtry.
Surely you don't actually believe that?
Because yeah it is!
If you do believe that, then whoever told you it isn't was lying their ass off to you, possibly to make you seem dumb. Unless you just assumed it isn't, because it absolutely 100% is a genetics based factor.
The genetics that decide this comes from both of your parents, half from your mother and half from your father. Which means yes it absolutely 100% is hereditary.
You have a 50% chance of taking after your dad's side of the family, & a 50% chance of taking after your mother's side of the family. But you also have to remember that your mother got half her genetics from your Grandma & Grandpa, same goes for your dad.
So if your fraternal grandpa (your dad's dad) had a small one,
and your fraternal grandma's dad (your great grandpa on dad's side) had a small one,
and your dad has a small one,
and your maternal grandpa (your mom's dad) had a small one,
and your maternal grandma's dad (your great grandpa on mom's side) had a small one there is probably like a 90-100% chance your gonna have a small one as well.
Which is just how genetics works, it's the same way for diseases as well, people can be genetically predisposed to certain illnesses like cancer, diabetes, etc.