To dimension a circle, select the circle at the first endpoint of the angle and then specify the second endpoint of the angle.
To dimension any other object, select the first line, and then select the second line.
Enter options as needed:
To edit the dimension text content, enter t (Text) or m (multiline text). Editing within or overwriting the brackets (<>) changes or removes the calculated dimension value. Adding text before or after the brackets appends text before or after the dimension value.
To edit the dimension text angle, enter a (Angle).
To confine the dimension to a quadrant, enter q (Quadrant) and specify the quadrant to measure.