Joe, Pip's loving, supporting, honest and good uncle, is one of the best men in the book "Great Expectations." Now, he isn't the "best" because he is the most intelligent, educated or refined, but because he has a loyal, true heart, and always was one of the few that treated Pip with love and respect before Pip was wealthy. Joe was a true friend to the mistreated Pip; he tried to soften the blows of critical adults that gathered at the house (remember the scene where Joe gave Pip extra gravy any time an adult insulted him? Pip had heaps of gravy to show for Joe's concern). He talked openly and honestly with Pip, housed many of Pip's secrets without disdain or reproach, and truly set an example of what a good man should be, in how they treat others.
So, Dickens makes Joe a pleasant-looking, large man. Joe is physically strong, parallelling his own strong character. Joe is big not only in body, but in heart, and big in influence over Pip. Even as Pip is ungrateful and rude to Joe, he always feels bad for it, and knows it is wrong. From the beginning, Pip doesn't want Joe to think poorly of him, and can't stand the thought of letting Joe down. This shows just how big of an influence Joe has been. So, Joe's largeness in size symbolizes Joe's largeness in influence and significance in playing the role of a kind, decent human being that loves Pip for who he is, not what money he holds. I hope that those thoughts help to get you started; good luck!