I like to size my ATOs to be between 8-9% of system volume. Or rather, i will have a larger ATO reservoir, but always keep it underfilled to this level.
Ill make a post on the specifics soon, but i started this rule of thumb for myself years ago. Essentially determined by the concern of: What if my ATO failed in the 'on' position, how could i reduce the risk of adverse effects to near 0.
Ill check my math before i make the post. That was a very long time ago.
Edit: and in terms of what to use, i usually make my own reservoir out of an appropriately sized plastic storage tote. Its cheap and has never failed me in 10+ years of multiple simultaneous tanks.