Sketcher > Dimensioning Sketcher Geometry > Working with Dimension Tools > Normal Dimensions > To Create an Arc Length Dimension
  
To Create an Arc Length Dimension
1. Right-click in the Sketcher window and choose Dimension from the shortcut menu or click Sketch > Normal.
2. Select one arc endpoint.
3. Select the other arc endpoint.
4. Select the arc.
5. Middle-click to place the dimension. The dimension is created.