Hi All... I'm a marine biologist so would hope to be qualified to answer your question! I'm not sure about an organism the size of a football
field - in fact, a single organism (especially a jelly) would probably not survive to that size anyway as they are so fragile. What you are probably
looking for is a colonial organism such as the siphonophores that have previously been mentioned. Physalia, the man-o-war is one, but there are
others that also get large. However, these organisms are mainly tentacles so are long and thin - you give the impression that your organism has a
large surface area?
I would suggest looking up 'salps' - specifically the Pyrosoma. These are colonial tunicates - related to sea squirts. Tunicates are actually the
most advanced of all invertebrates, despite their jelly-like appearance, and have a tadpole larva with a rudimentary spinal chord. They are,
therefore, almost vertebrates!! Some species of Pyrosoma (I think....) form rafts that may grow huge and be what you are talking about. Those that
form chains such as Pyrosoma spinosum may themselves grow many metres long. Hope that helps!!
N