I have found a fifty gallon to be a good size for an octo tank from the standpoint of maintenance (provided you don't have a bimac who grows extra large, like mine did- could have used a bigger tank, but we made do!).
A ten percent water change is one of the five gallon jugs that the LFS sells, and this quantity of water fits nicely into the big empty 7 gallon salt buckets. It's about the limit of what I can carry around. A larger tank, and I'd be using more jugs of water, more salt buckets, etc. Taking care of my 19 gallon invertebrate tank seems to be almost as much work.
Nancy