Arowana don't get 16 feet long but they do hit 3 feet plus pretty regularly so even a 125 is not big enough (how big is a 125 - 4 *2 * 2 feet or so?) They are not especially bendy, or good at turning round so you need at least 2 or 3 feet of width.
You see little rays that seem to top out at about 8 inches or a foot diameter disk, but for most it's worth assuming they are going to end up with a disk at least 18 inches across. People I've seen do well with rays have them in big, bare bottom tanks, and big means big, 6 feet by 4 feet base area, but they only need limited depth. Also note that rays are downright fussy w.r.t. water conditions so you might want to investigate plumbing the tank in as you're going to be changing a big load of water at least once or twice a week (think of 50, 100 gallon water changes.....)
you'll find big shallow tanks are pretty reasonably priced, but you do need somewhere to put them.