The guy is not standing upright, and the fish hangs not on the ground with the tail. Furthermore the guy is standing behind the fish, what makes it looking even bigger. This fish seems to be actually not longer than about 2,5m, most probably lesser. This is again a good example how big fish can look even bigger than they actually are.
I know that there are only "rumors" about those super colossal freshwater stingrays, but that´s the same with many of the other fish, including the arapaima (and I don´t believe in one of them, especially not in the case of the arapaima and the wels catfish, because I know they are false).
But anyway the giant freshwater rays grow very large. This guy for example could be well in the 3m-range and longer than every arapaima:
http://www.gofishing.co.uk/Netstar/...ageGallery/Thumbnails/59/30659/3240_11047.jpg