IT'S HUGE!: A biologist and broadcaster, Lizzie Daly, and her cameraman, Dan Abbott, spotted a giant barrel jellyfish over the weekend off the coast of England. Daly spent about an hour swimming with it. Barrel jellyfish start out small - about a millimeter in length and can grow up to 3 feet in length. They can weigh approximately 55 pounds when grown. Barrel jellyfish do sting, but don't quite pack a real punch.