Two lines built entirely on exaggeration: Jerry Hall is so small a rat could eat him whole, hat and all. The joke is in the hyperbole, best delivered with a grin so it lands as funny rather than alarming.
From Mother Goose / Nursery Rhymes (traditional). See the whole collection.
Jerry Hall, he was so small,
A rat could eat him, hat and all.
About this rhyme
One of the shortest rhymes here: two lines, and a single joke about how small Jerry Hall was. Good as a quick aside between longer rhymes, or a first rhyme for a very young child, since it takes seconds to learn.
Public domain. Text from The Real Mother Goose (Blanche Fisher Wright, 1916), via Project Gutenberg. View the source edition