Suppose a module is a perfect cillinder, 10x6 meters. Suppose the central section is 2 meters wide.

If you position the floors vertically (that is, each floor is the same size), each floor 2,5m in height, you can get four round floors, for a total of 99.39 m²

If you position the floors horizontally (as seen in the BA-2100 config), the first floor must be a few centimeters offset from the wall (otherwise you would be walking on the inside of a sloped surface).

Lets say the first floor is 20 cm offset. It will be rectangular and 20m² in area.

The next floor, 2.5m above, will lose a big part of its area due to the central core. It will consist of TWO rectangles, each one also about 20 m²

But you cant have a third 20m² floor, 2.5 meters above, for it will be only 80cm high!

In fact, even the first floor would have problems, for the central core would be 1,8 meters above the floor.

The best solution, for horizontal positioning, in terms of floor area, would only work for zero G configs (where floor area is not that important anyway, unlike in configs for lunar or martian bases)

Basically, you have two rectangular floors at each side of central 2 meters torus. Each floors will be 56m². But people´s heads will always turned towards the central core.

Between then, you have a lower ceiling floors, of 2m, divided by the central core in two. Disconsidering the area made unavailable by the central core, the total area would be around 30m² for this floor.

The total floor area thus would be around 142m²

Of course, its not even realistic imho to be talking about floor space for a zero g space station. It only becomes important if you are talking about a planetary station (or maybe, a spinning artificial gravity module for a space station or interplanetary ship).

In THOSE cases where gravity plays a role, and you have to measure in terms of floor space, it seems to me that at least in the smaller modules cases, the vertical floor setup (round shaped floors) provide most space.

If anyone can get me the dimensions of the BA2100 module, I can do some 3d graphs of the different floor configurations...