Click the Create
Dimensions icon 
on the Entities toolbar
or choose Symbols / Dimensions
from the main menu.
Pick the circle or double-click an arc to define its diameter.
Click the Create
Dimensions icon on the Entities toolbar
or choose Symbols / Dimensions
from the main menu.
Pick the circle or double-click an arc to define its diameter.
Position the dimension.
Edit its parameters with the help of the hot spots
and tips either directly in the graphic area or on the popup menu. Change font style 
and character size also, if required.
Press exit when finished.
Note: You can right-click anywhere in the graphics area to access the entity-specific and general functions from the popup menu:
Click each item on the Interaction Dialog for an explanation
Click the Diameter dimension parameters for an explanation:
Note: When using the ANSI standard, the angle of the witness line can only be changed within the circle:
See Dimension Overview for a general description.
Note: Spline approximation tolerance can be modified in Tools / Preferences / Drafting / General / Dimensions.