Jellyfish come in all shapes and sizes. Most jellies range from less than half an inch (1 cm) wide to about 16 inches (40 cm), though the smallest are just one millimeter wide! The largest jellies are the Lion’s Mane Jellyfish (Cyanea capillata), which on average are around 3 feet wide. Rare individuals, however, grow to be over 6 feet wide (1.8 m) with tentacles over 100 feet (30 m) long!