In Gary D. Schmidt's Trouble, we aren't told anything directly about Sanborn's size, but we are given clues to indicate his build is much bigger than Henry's, whom we know is on the small side because the narrator tells us in the opening chapter that Henry prefers crew to playing rugby since he is "one of the smaller rugby players" (p. 3).
The best clues we are given to show Sanborn is bigger than Henry are found in the moments they play around at beating each other up while waiting for Henry's mother to pick them up from school. Henry frequently asserts he could take on Sanborn, and Sanborn frequently retorts that Henry is dreaming. For example, the first time we witness Henry challenge Sanborn to a fight, Sanborn pulls two dimes out of his pocket and advises Henry to "make some calls to get help" (p. 39). The next time they challenge each other to a fight, Henry uses verbal irony to ask, "Wasn't it just a few days ago that I beat you up?," to which Sanborn retorts, "You must have been dreaming that, little man" (p. 94). Sanborn's choice to call Henry a "little man" is the best clue that Sanborn has the bigger build than Henry. At another time, Henry is surprised by how fast Sanborn pins Henry down with his face in the grass and Sanborn's knee digging into his back, another clue that Sanborn is bigger than Henry.