The only real reference I can find as to how Widge is described in terms of his appearance is actually on the very opening page of the book, where Widge talks about his background and childhood. Note how he is given the nickname by Mistress MacGregor of the orphanage that is used to refer to him for the rest of his life:
When she saw how small and frail I was, she exclaimed "Och, the poor little pigwidgeon!" From that unfortunate expression came the appellation of Widge, which stuck to me for years, like pitch.
It is clear therefore from this story that Widge is a boy who is smaller than his contemporaries and rather thin. The way in which Widge talks about food and how he and the other boys dream of it also reinforces the idea that he would be rather thin in terms of his physical appearance. We can conclude through this evidence that Widge would be a small boy for his age and very wiry and thin.