The opening lines of "How the Grinch Stole Christmas!" (Geisel 1957) set the tone for an inspiring change-of-heart story about a grumpy creature that comes to value humanity and the Christmas spirit. Although the book is often considered inspirational for other reasons, the authors were particularly moved by Dr. Seuss's interesting conjecture regarding the origin of the Grinch's bah-humbug spirit--the size of his heart--when he wrote, "May have been that his heart was two sizes too small." What does it mean that his heart was "two sizes too small"? What do students think of this question? This is the story of what the authors discovered when they asked middle school students what "two sizes too small" means. Their answers may change the way teachers think about teaching similarity. The authors begin this story by sharing their students' thinking about the Grinch's heart and how they answered the question. Interpreting what it means to be two sizes too small is difficult work, and the authors have found that it makes for a lively discussion about size and scale. (Contains 9 figures.)