IMO leathers can get any size. Theres not a whole lot stopping it from growing if the levels are right. Theres a dude on here (can never remember his name) who's had a fiji yellow leather for 19 years, named it steve, and it takes up more than half of a 90g tank. He's got pics of it on here too, i just cant remember the thread it is in....someone here will tho.
But IMO, it would be fine in a 20g tank. I see no reason you couldnt just frag it and trade off/sell the pieces you cut off if it gets too big for your taste.