The lion’s mane jellyfish (Cyanea capillata) is the largest among the jelly species, with the largest known specimen stretching across 120 feet (36.5 meters) from its top to the bottom of its tentacles. These tentacles contain large amounts of neurotoxins that can cause a range of effects when humans come in contact, from a rash to affecting respiratory function. Humans don't often come in contact with them because they tend to be found in the open ocean and not near coasts. However, they caused a famous literary death in Arthur Conan Doyle’s short story “The Adventure of the Lion’s Mane.”
Doyle did not write about another deadly jellyfish, one that is thought to be the smallest in existence. The irukandji jellyfish (Carukia barnesi) is dangerous even with a bell (the umbrella-like top of the jellyfish) only reaching about 20mm. The sting of this box jellyfish can cause headaches, chest pain, anxiety, vomiting, and occasionally fluid can enter the lungs and become life threatening. It is good to be careful around jellyfish of all sizes.