stairs and grid size

The main consideration is for ceiling height.

Floor area taken up by the stair is the second issue, both options are the same when having more than one flight of stairs, e.g. joining more than two levels.

I have chosen the 5x5x3.5 grid on this basis.

The option to have a single flight of stairs floor to floor has been rejected because the ceiling height of 2662 is insufficient.

The second option is the minimum space to contain a returning stair reaching 3500, allowing a ceiling height of 3000.