Area specs are not given by makers because it takes longer to maneuver around more furniture, making such measures unreliable, and instead give run time specs. They also do not care to expose the area facts. I have long been interested in the question.
The 960 runs 75 minutes vs 120 minutes for the 980. Another user reported the 980 required two charges to complete 1200 sq ft (with more tedious edge cleaning option engaged; the 2nd run could have been short, not reported). A Botvac
D5 has been reported covering 1300 sqft in one charge. The larger, wider intake Powerbot might do 1400 sq ft on one charge, faster. Bots which cover areas twice, crossing again at 90 degrees, will take longer and cover less -- examine detailed specifications.
Yet beware larger batteries will cost more, though with the newer lithium batteries could last a couple years or more.
Older random navigation bots will cover less than the systematic guided robots, taking more time to cover. They are now a lower cost category than the premium brands, with many offerings.
The lower end premium models are not oriented towards a large space and are supplied with smaller batteries (960 vs 980, Botvac
D3 vs D5). For such a large space even two robots might be appropriate, or doing half the house on alternate days. Batteries will wear out faster increasing long term costs. The specialty Dyson robot is oriented towards high suction power and small areas.