Does the size of the building have anything to do with the amount of time batteries hold charge for an inverter?
Absolutely not!
But that's not what you really want to know is it?
This is one of those questions we need to use our mind reading app here on DR1, Robert we will need more band width!
As soon as you said "two batteries", I knew what your problem is. Two batteries is not enough. Even new batteries won't go much further than 2 hours.
It is very Dominican to just add batteries just for the sake of adding batteries!