I have owned many oscars and a few Jacks over the years. My personal opinion is that A single fish should have a 75g minimum more than one 90g minimum. especially if you keep a scavenger or pleco with them. Both of the fish are awesome IMO the jacks get 8-9 inches as said, the oscars will push 11-14 depending on food, and genetics. 14" oscars are really rare, but they do exist. Some folks would argue that Dempsey's are fine in a 55, having kept them in 55's and in bigger tanks, I found the bigger tanks to be a much better idea. The jack is a little more aggressive than the oscar would be but either one can be quite territorial, and both are tough fish. either one will eat about anything, I used a couple types of prepared pellets, everything you can imagine in the frozen food department, and snails shrimp and guppies as occasional treats. The primary housing concern is that these fish need clean water and good nutrition. They will live in some pretty nasty tanks, but are succeptable to disease and problems in dirty tanks. couple that info with the fact that they are both messy eaters, and big waste producers and you need to be prepared for good maintenance and overfioltration to truly house them well. Both of these breeds are notorious for re-arranging the decor of a tank. Oscars especially will constantly try to re-position plants and rocks to their liking. This problem tends to be way less evident in bigger tanks IME, this is just one more argument for keeping them in larger volumes.
Dave