The only situation that sounds similar is acromegaly. However, this causes excess bone growth in addition to soft tissue. So the hands and feet enlarge as well as the bones in the face causing the face to be disfigured. The complications are horrible including heart disease, arthritis, diabetes, and premature death.
The problem is that the body is an entire system. It is very hard to target just one organ or specific area of the body chemically. Even most medicines that we use today have side effects that affect multiple organs.
The reason I was teasing is because of the way the guy wrote the letter. It smells like total bull. If he had rewritten it to state that instead of a large penis, he had instead won millions of dollars and his wife can’t deal with it and blah blah blah it still sounds like a “ladies here I am” letter.