Tried and and was able to change the diameter. However the start and end symbols of the walk line appear fixed and can't move, so the walk line is off centered and not much use. It appears that the tread width is measured at the walk line. If you change the tread dimension in the settings it can be measured at the walk line as the tread dimension setting.
I think that the custom dimension in the walk line floor plan display setting is locked to center of stair width or grayed out and not available for a spiral stair. Shouldn't be this difficult.