Shreks genetalia most likely correspond to his large body size. One can imagine that his weapon of choice is very large, especially if he's meant to please Fiona with it. The official answer ranges from 12 to 14 inches in length and about the width of a cob of corn in diameter. Noncanonically however, most fans tend to fantasize that Shrek's penis approaches 20 or so inches. For the short time in Shrek 2 after Shrek consumes the "Happily Ever After" potion, he becomes the ideal man. During that period he may be closer to 10 or so inches.