In the context of this quote, Montag is reflecting on how interesting, odd, and thought-provoking his encounter with Clarisse had been. Montag uses a series of metaphors and similes to describe her. As Montag struggles a bit to describe how interesting Clarisse is, the narrator even states this directly, saying that Montag was "searching for a simile." He thinks of how other people are nothing like Clarisse; other people, most people, are more isolated and non-interactive:
People were more often--he searched for simile, found one in his work--torches, blazing away until they whiffed out.
Metaphor, Personification, and Synecdoche:
But in this quote, "what a shadow she threw on the wall with her slender body" the usage is more of a metaphor than a simile (which tends to use "like" or "as" in comparing two different things). Clarisse is a small girl but casts a big shadow. One could take this literally, meaning that because of the lighting and angle of her body, there was a large shadow on the wall. But this is certainly also a metaphor meaning that Clarisse is a small person but her personality and/or her affect on Montag was quite large and significant. You could also say that her shadow is personified (another figure of speech) to mean her personality itself. Clarisse casts a large shadow on the wall just as she casts/affects her personality (herself) onto Montag. Her shadow represents her own personality; her shadow is, therefore, personified.
One could also say that synecdoche is used in this quote. Synecdoche is used when a term for a part of something is used to represent the whole thing (or vice-versa). However, this is a bit of a stretch because saying that her shadow (metaphor for her personality) is a part of her is dubious. Still, you could make the argument that her shadow is a metaphor for her personality, the shadow is thus personified, and the shadow (being a part of her in a sense) is a symbol for her whole being, via her personality.