It turns out that Hitler's medical records show that he had some serious issues with his manhood. New documents have been discovered that go into more detail. Historians Jonathan Mayo and Emma Craigie wrote about it in their new book Hitler's Last Day: Minute by Minute which was quoted by the Telegraph:
Hitler himself is believed to have had two forms of genital abnormality: an undescended testicle and a rare condition called penile hypospadias in which the urethra opens on the under side of the penis.
According to The Telegraph, hypospadias often leaves the sufferer with a micro-penis. So, there you have it.
They say size doesn't matter but that hasn't stopped the internet from speculating.