Prescott is used to answering that question. "I'm still growing," he insists. "I'm 12 and a half stone now. When I started out playing for St Helens in the Academy team I was only 10 stone four."
He works harder than any of his Saints team-mates on his weight training; he shovels down his food and his weight-gain supplements. And yet there is still a deceptively frail look to him and a temptation to cover your eyes with your hands when he is confronted by a Meninga or a Tuigamala travelling towards him at full speed.
There is no question about Prescott's courage. He meets the big men head on, with no self-doubt, and knocks them down.
By its very nature, however, the rugby league full-back's job sometimes involves being left in a hopelessly exposed position, where you can do everything according to the book and an opponent's momentum will make him unstoppable.
When that happens, men turn to each other on the Knowsley Road terraces and say: "There you are. Not bloody big enough." It happened a couple of times on Boxing Day, when he celebrated his 21st birthday by losing out to Wigan in a thrilling Bank Holiday derby.
"I dropped a ball as well," says a young player whose own self-critical faculties are sharpened by having a father, Eric, who was a distinguished player, and now "points out everything I've done wrong, even when I think I've had a good game."
Eric Prescott, a rugged second-row with St Helens, Widnes and Salford, is one of the historical precedents from which his son draws encouragement. "He started off as a little winger and finished up in the front row," he says.
The other is the one player he regards as a model - Jonathan Davies: "He isn't much bigger than me. When he plays full-back, we are virtually the same player - same pace off the mark, same side-step."
Prescott, even in his first full season in first-team rugby, does have much of Davies's invigorating approach to the game, and playing full-back in an expansive side like St Helens gives him the opportunity to attack explosively from deep in his own half.
That experience has left him in no doubt that full-back is his best position: "I started off with a few games on the wing, but I didn't like it very much," he says. "At full-back, you've got so much space. You can look for the gaps and pick your moment."
Prescott's moments to remember during his first year in first-team rugby have been numerous. "It has been a step up from the A team, but you get used to it," he says. "You put yourself in another gear, just as you have to do when you make the next step up to international football."
He has already had a brief taste of that, playing admirably against Australia in Gateshead for a well beaten, but brave and resilient Great Britain Under-21 side in November. "We had them going at one stage," he says, in another mark of the self-confidence he brings to his game.
Further promotion might have to wait, although it would be no major surprise if Prescott was to figure in a full Great Britain squad before the end of this World Cup year.
For the moment, though, his concentration is firmly on cementing his place in a St Helens side which feels that the time is ripe for winning something - if not the Championship then one of the two knock-out competitions.
There will be days when the men on the terraces will wish out loud that he had inherited more of his dad's bulk, but Prescott has the skill and determination to win through.
There is a story, too, from which he might draw extra encouragement as he reflects on the birthday present that Wigan gave him.
In another meeting between the two sides a few years ago, another lightweight St Helens full-back had been given a torrid time in the first half.
"Bit of a rough half for you, eh?" said one of Wigan's senior players, with every show of sympathy. "Don't worry about it," he told a young man grateful for the surprising gesture, "because in the second half we're going to bloody kill you."
The shell-shocked youngster was Gary Connolly, now of Wigan and Great Britain, into whose face no one has kicked sand for quite some time. Steve Prescott has the ability to make it just as big.Reuse content