Wow. Only two jellyfish in the world get this big, Nomura's Jellyfish of Japan (Nemopilema nomurai) and the Lion's Mane Jellyfish found in Arctic and Australian waters (Cyanea spp?). 'Spp' stands for species plural; because of size and color variation across its range the Lion's Mane is not known to be a single species. I'm pretty sure the above image is of Nemopilemna nomurai. The Lion's Mane (which is the larger of the two) has a bell divided into eight lobes as seen below.
If you define 'plankton' as pelagic sea creatures whose movement is largely guided by the currents, these are the largest plankton is the world! The record holding Lion's Mane specimen had 120 ft tentacles, longer than a Blue Whale. Still, they have to worry about jellyfish predators like the equally massive and comically proportioned Ocean Sunfish (Mola mola).