Circular / Round Stair Case
The code requires certain minimum and maximum dimensions for spiral stairs. 

The Code defines the “walk line” as a distance 12” from the narrowest portion of the stair AND the tread must be a minimum of  7 1/2” wide. (See ICC R311.7.9.1)

The riser shall be uniform in height and a maximum of 9 1/2”. 

The width of the stair shall be a minimum of 26” wide below the handrails. That is to say, the handrails cannot protrude into the 26”, but you can make the stairs wider and have the handrails protrude into that space. (See ICC R311.7.9)

Also, there shall be a minimum of 6’-6” headroom. 

Handrails shall be not less than 30” above the nosing of the tread and not more than 38”. (See ICC R311.7.7.1)