"David's size was notorious, and he loved to pull it out with both men and women. It was so funny, adorable and sexy."
Harry went on to explain to The Sunday Times how she assumed Bowie was revealing himself to her as a show of thanks for supplying the drugs.
"It was very funny… I didn't touch it. But I did think, well – very nice. I don't know, it's too bad you can't ask him," she said.
"I guess I was sort of flattered, you know? He's one of the great men that I admire in the music world, clearly a genius."
The book includes Harry detailing other episodes in her life, including that she was raped at knifepoint in the 70s, and how she was lucky to evade being kidnapped by notorious serial killer Ted Bundy.
Harry explained her assault occurred before she was famous, saying she and her then-boyfriend Chris Stein were tied up by an armed robber at their New York home before she was raped.
"He poked round searching for anything worth anything," she said. "He piled up the guitars and Chris's camera and then he untied my hands and told me to take off my pants.
"He f**ked me. And then he said, 'Go clean yourself'."
However, she played down the brutal attack, saying: "The stolen guitars hurt me more."
Harry went on to explain how she narrowly avoided being captured by the man she believes was Bundy, saying: "I got in [his car] and the windows were rolled up except an inch and a half at the top. I saw there was no door handle, no window crank, no nothing. The inside of the car was totally stripped out.
"The hairs on the back of my neck just stood up."
She put the window down and opened the door from the outside, explaining: "He tried to stop me by spinning the car, but it helped me fling myself out. After I saw him on the news. Ted Bundy."
Bundy went on to murder 30 women before he was executed in 1989.
Harry will today set out on a tour to promote the release of her new book, beginning in New York and finishing in Miami on November 20.