Look at the grainy underside of the jelly. You can really see the pixelation where it's dark, as if it was blown up. You cannot, however, see any such pixelation on the diver. Also, look at the edges of the thing the diver is holding, it looks slightly pixelated, there, too.

With that said, th elargest jellyfish are the Arctic Lion's Mane, which grow to 2m across the bell and 36m long tentacles. Judging by the diver, that jelly is at least 30 feet across.

The jelly in the picture is not,as far as I can tell, an Arctic Lion's Mane, though it may be a Cyanea.

Actually, the Portuguese man-of-war is not a single marine animal, but it is a colonial hydroid, consisting of a large colony of smaller marine organisms. The blue-bottle gets its name from the body, which really is a large, gas- filled, bladder-like blue float, that can be up to 30 centimeters in length and rise above the water as much as 15 centimeters. Clusters of colonial polyps are attached to the underside of the blue bladder, and very long tentacles (sometimes up to 10 meters in length) extend from each of the polyps.