There's no real rhyme or reason to the verse form in "Cinderella," but in terms of syllable count, Sexton is remarkably consistent. If you count all the syllables in any given line, you'll find that nearly every line falls between seven and twelve syllables. This gives the poem a consistent feel from line to line, and doesn't distract us from the plot, which is the main focus of the piece.
The stanzas vary greatly in length, from a mere five lines to giant blocks of over 25 lines. Each stanza is a separate plot unit (notice how they all end in complete sentences) and serves to separate the story up into manageable chunks. The fact that they're all end-stopped (meaning they end with a period) gives the reader a chance to pause a bit between stanzas and helps create the clipped, matter-of-fact tone that infuses the poem.
In fact, there isn't much in the form or rhythm of this poem that could take the reader away from the content itself. Formally speaking, there is one unique line of just one word: "Once" (line 22). That can't be an accident, so it's worth a mention (which we do in the "Line by Line" section).
There's just one other formal consideration to make, though, and that's the use of the refrain. Do you notice any repeated lines in this poem? Oh, right. The refrain. "That story." The same two-word line appears at the ends of stanzas 1, 2, and 4, as well as at the end of the poem itself. It's as though Sexton is reminding us that the old, well-known fairy tale of Cinderella ("That story") is not quite what we all thought it was, and this poem is here to set us straight.