There is not a lot of text devoted to the specifics about Jeffrey "Maniac" Magee's physical appearance.  Here are about the only two descriptions of Maniac Magee. The first one is when he arrives in Two Mills and the second one is McNab's thoughts about Maniac before trying to strike him out.  

"a scraggly little kid jogging toward them, the soles of both sneakers hanging by their hinges and flopping open like dog tongues each time they came up from the pavement"

"Just a punky, runty little kid." 

At times, I don't like it when an author doesn't give much of a physical description.  I want to know what a character looks like.  I want to know what I'm supposed to have in my head.  But for "Maniac Magee," I'm glad that the author didn't spell it out for the readers.  Maniac is supposed to be a legend.  He did the cool football catch and punt ... one handed.  He hit home run after home run off of McNab.  He ran away from home to Two Mills (200 miles).  He is "cool times ten."  

The reader is given just enough physical description about Maniac to know that his feats of awesomeness shouldn't come from someone who looks like him--a skinny, short, runt of a kid.  A young kid too.  Eleven years old and he is besting high school athletes.  You, the reader, are meant to build this legend of a scraggly 11-year-old in your mind.  His mental image of actual appearance will differ from person to person, but that doesn't matter, because what makes Maniac great isn't how he looks.  It's how he treats others.  It's how he acts.  It's his heart and attitude that matter. That's what makes him a legend.