In bits and pieces, Max describes his physical appearance in the first several chapters of Freak the Mighty. On pages 3 and 4, Max overhears Grim and Gram talking about how Max resembles his father. On that same page, Grim also jokes that Max is growing so fast that they should “let [Max] go barefoot, he’s exploding out of his shoes” (4). So at this point in the novel, Max hasn’t described himself, but we know from the commentary of others that he is big and looks like his father.
In fact, as the narrator, Max spends most of his time describing the physical appearance of others, especially Freak and his mother Gwen, in the first chapters. It isn’t until after Gwen is startled by Max that he describes his physical appearance. Philbrick writes (23-24):
Gram reaches out and she puts her hand on my shoulder, real light and feathery, you can feel how nervous she is just to touch me, and how it makes her uncomfortable to have to look up at me, because did I mention I'm a lot bigger than Gram? Bigger than Grim, too? Bigger than most people? It's true.
Max then tells Gram that his physical size must have intimidated Gwen. Max seems to be used to this reaction from others. When Max goes over to Freak’s house for dinner, though, we realize what really scared Gwen was the fact that he resembles his father.