There are plenty of “giants” that Leslie and Jesse face in real life in the Bridge To Terabithia, but one, in particular, comes during a chapter called “The Giant Killers.” The chapter explicitly mentions that “Leslie liked to make up stories about the giants that threatened the peace of Terabithia, but they both knew that the real giant in their lives with Janice Avery.” (Chapter 5).
While you could talk about giants being the challenges that Leslie and Jesse have to overcome, the book puts forth Janice as one of the real giants. Janice Avery was a bully, and in chapter five she takes advantage of May Belle, Jesse’s younger sister, and steals her twinkies. May Belle demands that Jesse fight Janice, and she calls him “yeller” for not wanting to do it. Jesse is fearful of being seen as weak and afraid, but Leslie comes to the rescue.
Leslie tells May Belle that Jesse can’t physically fight Janice because he would get kicked out of school. Instead, Jesse and Leslie devise a plan to get back at Janice by making a fool of her, something she hates. So they decide to leave a note from her crush Willard Hughes, telling her that he wants to walk home with her and talk about “us.” She, of course, has to walk home alone and is deeply embarrassed. That is one of the real giants that Jesse and Leslie have to face. Eventually, they become somewhat friendly with Janice, but it takes some time.
Other possible giants the kids face could include their hard relationships with their parents, Jesse facing up to his love of drawing in the face of toxic masculinity, and eventually Jesse facing the fact that Leslie died while he was away having one of the best days of his life. Those are much deeper metaphors than the giant of Janice Avery.