I had one. I had a male Abdopus aculeatus. His mantle was about 2 inches long and his longest arm was about 12-14 inches. He was VERY active and friendly once he got over his fear of people. It was difficult to keep him mentally stimulated, but plastic cat toys, glass cups and opaque coffee mugs fascinated him.
Unfortunately, he only lived for two months because I bought him as a fully grown individual, and dwarf species like aculeatus generally live for less than a year anyway.
I kept him in a 30 with no problems. I did weekly 10 gallon water changes. Dwarf species are generally considered able to be kept in a 30, but sumps are still recommended. I'm going to have one before I get my next 'pus.
I was truely sad when he died because he was interactive and intelligent, like a dog. He was a cool pet, not just some critter living in a tank.