As a title, "The Great Figure" is a big mystery. The noun "figure" can simply mean "number," like when you multiply two figures together in math class. It can also refer to a person's body, like "No dessert for me today. I need to watch my figure." But in poetry, a "figure" is also a word or sign that represents something else. "Figures of speech," like metaphors and similes, are the bread and butter of poets. So when a poet like Williams highlights a word like "figure," you know he can't just be talking about numbers.
At a literal level, "The Great Figure" refers to the number 5 on the fire truck. But what makes this figure so "great"? Five isn't exactly a huge number, and the speaker even points out that the number goes "unheeded." On the other hand, maybe the speaker's attention to the number, expressed through the poem, is what makes it "great." People can use their imaginations to turn trivial details into great things…or something like that. Or maybe there is some other "figure" in the poem that we're unaware of. Maybe the whole poem is a "figure" that represents something else outside the poem. If you ask us, Williams is trying to be mysterious, to make us think about why we call things "great" or "small."