Hugh Hovey hosted a series of fan-made but author-endorsed schematics. From this we can deduce that the staircase isn't spiralling around the outside, but is rather more central. The cross-section has each floor being around 200+ feet wide, but there's no indication of height (or rather depth).
We do know, based on the fact that several of the "floors" have hidden bunkers and/or are able to host large equipment like power generators, that they're not just single-sized floors (like you might find in a skyscraper) but possibly each landing could well be leading up and down to sub-floors, or just have very high ceilings. That would explain why crossing a set of 30-40 floors can take a whole day if they're having to head down the equivalent of 5-10 floors per 'level'.
‘Twenty floors in just over two hours. Don’t recommend the pace, but
I’m glad we’re this far already.’ He wiped his moustache and reached
around to try and slip the canteen back into his pack.
Wool
Assuming a sedate walking pace, gives us a rate of about one level every six minutes. That would imply that each floor has a depth of about 80-100 feet, and the Silo a total depth of 12-15km, not accounting for the mines under it.
This is backed up in the sequel novel Dust. What we would consider to be a normal 'level' (roughly the height of a building floor) is considered by our heroes to be less than half or a quarter the height of a level in Silo 17.
They had explored the bunker down to the bottom – it was only twenty
levels deep, and the levels were so crazily packed together that it
was more like seven levels tall.
This also doesn't taken in account out-sized levels like those found in the Engineering and Mechanical districts.