Space is measured in RSF: rentable square feet. This is a number that bears only a fleeting relationship to the actual size of the space. I have seen spaces that are 1000 RSF which are the same size as spaces that are 3000 RSF.
How could this be, you ask?
Well, you don't want to know. Officially you get RSF by taking the total size of the space and multiplying it by the building's loss factor. That's a made up number which they multiply the size of the space by, just because they can get away with it.