Most leeches are between 0.75 inch and 2 inches (2 and 6 centimeters) in length, but some “medicinal” leeches reach 8 inches (20 centimeters). The giant of all leeches is the Amazonian Haementeriam ghilanii (from the Greek term haimateros, meaning “bloody”), which reaches 12 inches (30 centimeters) in length.