Maybe a 250g, but JMO and many peeps keep 'em in smaller tanks. I figure, if they reach 1 foot, that only leaves 5 ft to swim and not a lot of turn around room in a 125g. Of course, they take years to reach that size, and that is why you see 'em in smaller tanks all the time.
Also, JMO, if I had a big enough tank, and was gonna get a large tang, it would be a naso. I don't like the hippos as much once they reach about 7 or 8 inches. They look a lot nicer when they are smaller, in my opinion. A nice big naso with streamers, is a much nicer looking large fish.
Also, only giving my opinion cause you asked. It's your tank and a hippo would do fine in a 75g for a couple years(esp. if you got him real small), but I don't think he would last to full size in there.