The best place to look for direct characterization details regarding what Kevin looks like in Freak the Mighty is in chapters 1 and 2. Unfortunately, readers don't get a lot of details on exactly what Kevin looks like. Max is the story's narrator, so we see things from his perspective. Additionally, Max isn't a character that is usually great at explaining things at length or in detail. Consequently, his descriptions of Kevin's appearance are repetitive and fairly vague. Max will say that Kevin looks "weird" and "twisted" rather than fully explain his observation.
Chapter 1 has Max and Kevin in day care. Max tells us that Kevin looked small, but it wasn't that big of a deal, because everybody was small. We are told that Kevin had crutches at first and then a bit later leg braces. Kevin's attendance is sparse in day care, and the next physical description of Kevin occurs when Max is in third grade. Max tells readers that Kevin has yellow hair.
In chapter 2, Kevin and his mom move in next door to Max, and readers get a bit more detail on what Kevin looks like. We are told that Kevin looks like a "midget kid." His yellow hair is on a "normal-sized head"; however, the rest of Kevin's body is "shorter than a yardstick and kind of twisted up." The twisting prevents Kevin from standing up straight and causes his chest to be puffed out. From that point forward, readers get narration about the things Kevin wears and does, and those kinds of things help readers see who Kevin is rather than what he looks like.