The actor revealed he was "not extremely well-endowed" when asked about losing his virginity in a 2009 interview with Playboy.
But LaBeouf need not have feared failing to measure up on set, as digital technology was on hand to create the graphic sex scenes that characterise the movie.
Last May, producer Louise Vesth told the Hollywood Reporter: "We shot the actors pretending to have sex and then had the body doubles, who really did have sex, and in post (production) we will digital-impose the two.
"So above the waist it will be the star and then below the waist it will be the doubles."
Vesth added that the depictions of sexuality throughout the film go "right up against the censorship limits, maybe just over them".
In 2012, LaBeouf told US chat show host Chelsea Handler that he had sent von Trier a sex tape to secure his role in the production: "I sent him videotapes of me and my girlfriend (then stylist Karolyn Pho) having sex and that's how I got the job," he said.
The Transformers actor has gone full-frontal in the past when he lent his nude assets to Icelandic indie band Sigur Ros' Fjogur Piano music video.
Charlotte Gainsbourg, Uma Thurman, Jamie Bell and Stacy Martin are among the Nymphomaniac cast. The film was promoted with a risque poster campaign depicting images of the ensemble pretending to orgasm in October 2013.
Nymphomaniac premiered in Denmark on 25 December and is due for UK release on 21 February.
- Independent