Hope you are on version 11...
go to design menu, choose stairs.
On stair options choose the spiral.
On the spiral panel you can choose the size, controling hor. & vert. dimensions on the right side of the panel.
Control the size of the center on "stair well diameter"
Play with different options especially "number of risers vs tread height" to assure proper stair height and correct tread & risers sizes.
Warning- Seldom you will get the stairs exactly as you wish. Be satisfied with an aproximation. Most of the times youll need to add extra 2d linework (lines, empty fills) to correct stair plans and elevations.
