The physical appearance of Max is described on page 26 in my edition, when Gram goes to Max and tells him that Gwen is sorry for what she said about him and also that she is delighted that Max and Kevin are going to be friends. When Gram reaches down and touches Max on the shoulder, Max mentions the first clear indication that we have had about his personal appearance and what he looks like:
...did I mention I'm a lot bigger than Gram? Bigger than Grim, too? Bigger than most people? It's true.
Max therefore explains why it makes some people nervous to be near him, because he is so much bigger than other people, which makes others think that he is a figure to be afraid of. It is his size which apparently intimidates those around him and makes them slightly uncomfortable.