A gross area measurement isn't really helpful for that kind of determination.
The dimensions of the floor, the ceiling height, distance to the closet, walls, elevator placement, etc are all much more useful.
Even with all the above (or more), the only way to properly make that kind of determination is with a site survey.
If the APs will be ceiling mounted near the center of an area, you'll get better coverage (~twice the area) than if you wall mount. If wall mounting is your only choice, then you should be looking at some flavor of patch antenna (hemispheric covererage).
There are many variables. You can probably "ballpark" a number from a floorplan, but, as mentioned, a site survey is your only guarantee of performance.
Good Luck
Scott
