To Dimension the Size of an Arc

Select the Angular Dimensioning tool.
Set the mode to Arc Size.
Enter a data point to define the dimension start point. The dimension is measured in a counterclockwise direction from this point.
Enter a data point to define the endpoint of the dimension.
Enter a data point to define the length of the extension line and the radius of the dimension arc.
Do one of the following:

Go back to step 4 to place another dimension using the same start point.

or
Reset to complete the dimension.

Dimensioning the Size of an Arc