Not to take sides, but on a conceptual level, I'm with Myka on this. A smaller reservoir is less of a potential point of failure than a large one.
Why ?
We use technology to simply the complex tasks; the simple ones can be left to the dumb humans. The instructions to the warm body checking your house daily is "Take this jug of water and use it to keep that reservoir full". If they can't handle that then perhaps they shouldn't have a key
Consider the 2 possiblities with respect to the ATO and its reservoir:
1) Small Reservoir
a) Float failure and ATO pumps entire contents into tank. If properly sized, this should not create an overflow situation and is unlikely to decrease salinity to a dangerous level. The caretaker should raise the alarm on a suddenly empty ATO reservoir after a single day.
b) Float Failure of a different nature and water is never added to the tank. Well this should show itself in diminishing sump levels and the caretaker should raise the alarm that they DIDN'T have to put any water in the reservoir.
2) Large Reservoir
a) Float failure and ATO pumps entire contents into tank. Well...big mess; major salinity drop...tank crash
b) Same as above
It's worth noting that a float switch in your sump can be used to warn of situation 'b' (for both reservoir sizes).
All that being said, if your reservoir is REALLY small and needs to be filled daily then you have to ask why you'd spend money on an ATO in the first place.
Somewhere in there is a reasonable compromise and I'm guess it's in and around the 3-4 days worth mark.