Apparently, the “small penis rule” is a sneaky defense strategy authors can use to save themselves from libel lawsuits. Here it is described in a New York Times article from 1998:

...For a fictional portrait to be actionable, it must be so accurate that a reader of the book would have no problem linking the two,” said Mr. Friedman. Thus, he continued, libel lawyers have what is known as “the small penis rule.” One way authors can protect themselves from libel suits is to say that a character has a small penis, Mr. Friedman said. “Now no male is going to come forward and say, ‘That character with a very small penis, that’s me!

Now, from Wikipedia:

The small penis rule was referenced in a 2006 dispute between Michael Crowley and Michael Crichton. Crowley alleged that after he wrote an unflattering review of Crichton’s novel State of Fear, Crichton libeled him by including a character named “Mick Crowley” in the novel Next. In the novel, Mick Crowley is a child rapist, described as being a Washington-based journalist and Yale graduate with a small penis.

I did not know about this. I guess you do learn something new everyday.