It's possible you're thinking of some of the vivid imagery and descriptions that, while not strictly idioms, might feel like them. For example:
* "He was as thin as a rake" (describing James) - This is similar to the idiom "thin as a rail".
* "The peach was as big as a house" (describing the giant peach) - This is a hyperbole, a figurative language technique often used in idioms.
If you can provide a specific instance from the story, I can help determine if it's an idiom or a similar figurative language technique.