Your bimac will enjoy living in a large tank!
THis is what you find in Cephalopods: A World Guide
Body 5 inches, arms to at least 14 inches
However, we had some that grew considerably larger. My bimac Ollie weighed over three pounds at the time of her death, had a mantle more like 7 inches and arms that were more like 20-22 inches. It's hard to measure an octopus because they change sizes so easily.
I think the reason she grew so large was heredity and also very good quaility food including live food every day.