The picture above isn't mine, but it's what I envision him looking like whenever I read CraferLily's stories or I write him in my own things.

He actually has really big white eyes in the stories but he isn't possessed or corrupted by Herobrine. He is small. And yeah he looks like Link from Legend of Zelda, but that's just how it is. He is not related in any way to Link or anyone else from the LoZ.

So for size, I'd reckon chest size when standing next to someone like Nick or Grayson.

And he goes pretty crazy when in battle, so for all you want you can draw him wearing MLG glasses while taking down a monster that is more than four times his size.