Size Him Up site asks users to input the size of the man's hands, feet and nose.
The supposed length of the man's penis then appears. According to the Metro newspaper, the site says David Beckham is a "small" 5.2 inches.
It says Dermot O'Leary is "medium" at 5.9 inches while it measures swimmer Ian Thorpe at an "extra, extra large" 8.3 inches long.
A spokeswoman for site owner and condom manufacturer Condomi said: "It has been scientifically tested and proved to be accurate."
She refused to give details of the formula it uses to work out its answers.
