average disc size is roughly 14 inches, off the top of my head. you'll of course see variation in it, but i wouldn't think to a massive extent. i don't think they would get anywhere near hitting 20 inches, especially in an aquarium environment
ive got my trio in a 210 long. its an 8x2 footprint. its only them in there they are doing great. gonna do some hydro cycles soon and get some pups going!
My 1 female is around a 16" disk diameter. I have heard from 4 peoplle now she is the biggest hystrix they have ever seen. She makes the tank look small and pics just don't do justice.