Allen is now a £15 million player at Anfield.
He told Liverpool FC magazine: "It (height) might have been an issue for other people but never for me.
"It was something I had to fight against but I wasn't really bothered. I kept chasing my dream, believing in myself. You have to be single-minded.
"You could ask anyone in this dressing room and I'm sure they'll tell you that there were people that doubted them. That's why they're here now, I guess; because they had the mental strength to prove the doubters wrong. I think football is a game where there is a style of play or position for anyone.
"I think talent is essential in football. On a football pitch it is the technique of a player that always makes the difference, not his physical power."