Octopi are fascinating critters. They have advanced brains that can figure out unscrewing a jar lid to get a treat, and they have uncanny skills escaping aquariums and even sliding away to drains and back to the sea, in some cases. I saw a factual documentary about a man and his daughter who kept an octopus in a large home aquarium and really got to know it, and it got to know them, and it was quite interactive with them and aware of their doings.
I'd guess it was about three feet long all stretched out, though it usually wasn't. The tank was very large for a living room, about six or eight feet.
I'm not sure about the really large octopi, but this one was typical in having an entire lifespan of just one year, so they went through losing it, but knew to expect it. As advanced as they are, it is interesting that they have such a short lifespan. They have to develop all of their considerable skills in weeks.
Also, anatomically, their eyes are as sophisticated as human eyes, though they developed independently on an entirely different evolutionary branch, just developing many of the same structures and functions.
As with the jar unscrewing exercise, they know exactly what they are doing with all eight tentacles at once and have a highly tuned sense of touch.