Even though most people call them jellyfish, we call them jellies because they aren't actually fish. They are marine invertebrates so they don’t have a skeleton.
As far as what is the “biggest” jelly, it depends on how you define it. If you mean the longest, then the lion's mane jelly (Cyanea capillata) has been known to have tentacles about 30 meters (100 feet) long, which makes it one of the largest animals in the ocean, about the same length as a blue whale. It also might have the largest bell diameter. Although normally only 1.5 to 1.8 meters (5 to 6 feet) across, there are reports of some individuals with bells of 2.7 meters (9 feet) in diameter. The lion’s mane jelly is found in cold northern ocean regions, so you may find one in Alaska’s coastal waters.
If you’re curious about what is the biggest by mass, then there are jellies like the nomura jelly (Nemopilema nomurai) that can be 1.8 meters (6 feet) across and weigh more than 180 kilograms (400 pounds). Most large jellies in the ocean are not collected and weighed so there may be heavier jellies in very deep water.
I personally would say lion's mane jellies take the title even though their bodies are thinner. Keep in mind, though, that jellies don't have to be big to have a very strong sting so you have to be careful whenever you encounter one!
— Vince Levesque, aquarist, Birch Aquarium at Scripps