In this image of Scylla by John Flaxman, you can see that she has grabbed up some of the companions of Odysseus in her hands, which gives you a sense of scale. This Scylla is a gigantic monster! Flaxman has chosen to depict Scylla as still recognizably human from the top up; in other accounts, she is supposed to have six heads - not just one. Down below, you can see that she has tentacles instead of legs, with barking dogs' heads around her waist. You can read more about Scylla's appearance in this Wikipedia article.
Scylla posed a threat to sailors on one side of a narrow sea strait (perhaps the Strait of Messina between Italy and Sicily?), while the whirlpool monster, Charybdis, threatened from the opposite side. Sailing "between Scylla and Charybdis" was one of the perils that Odysseus faced on his journey home (see The Odyssey, Book 12). Compare also the Latin saying, Evitata Charybdi in Scyllam incidi, "Having avoided Charybdis, I've fallen into Scylla" (Erasmus, Adagia 1.5.4) - something like the English saying, "out of the frying pan, into the fire."