Cyanea capillata carries up to about 1,200 tentacles arranged in eight clusters beneath a bell that can exceed two metres across in the coldest waters. Bell diameter is the dependable measurement. Tentacle length is not, because the tissue is elastic and a drifting animal's threads extend and contract with current and handling.
The famous figure comes from a single stranded specimen described from Massachusetts Bay in 1870, whose tentacles were given as roughly 120 feet. That measurement has been repeated for over a century and is the basis for the claim that the species outreaches the blue whale. It has never been matched by a comparable modern record.
Size tracks temperature clearly. Individuals in Arctic and sub-Arctic water grow far larger than those in temperate seas, where bells of tens of centimetres are ordinary. The sting is painful but rarely dangerous to healthy adults, and detached tentacle fragments in the water or on a beach still fire their stinging cells, which is where a large share of stings actually comes from.