In an open
sketch, click Smart Dimension
on the Dimensions/Relations toolbar, or click Tools,
Dimensions, Smart.
Select the circle.
You can create a circular dimension as a diametrical dimension or as a linear dimension. By placing the dimension at an angle, the dimension is displayed as a diameter. By placing the dimension vertically or horizontally, it is displayed as a linear dimension.
To create a circular dimension and then rotate the dimension around the circle:
In an open
sketch, click Smart Dimension
on the Dimensions/Relations toolbar, or click Tools,
Dimensions, Smart.
Select the circle.
Drag the dimension and click to place it.
Set the value in the Modify
box and click 
.
If the Modify box
does not appear, either double-click the dimension or select Input
dimension value in Tools,
Options, System
Options, General.
If the dimension is a diametrical dimension, select the dimension drag handle and drag to rotate the dimension to reposition it.