Like others said, depends on different factors. Evaporation from heaters during winter and fans during summer. Go the biggest that you can go with that fits in your stand, or can build matching enclosure next to it. If you are gone longer, just have separate container you only use for extended periods.
Usually RODI can be plumbed directly to ATO. Can put it on timer, or have it connected to controller where you can access remotely to turn on. Before, I just had colleague turn on and off for me once a week. I think I can go 2-3 weeks with a 40g brute.
Had an oopsie not because ATO failed, but skimmer cup overflowed and leaked water out of the sump while ATO keep putting water in. (There was carpet cleaners at office when I was gone and chemicals made skimmer nuts). Added float switch to skimmer cup.
Other limitations are refilling dosing containers, emptying skimmer cup, auto fish feeder.
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