Arc Dimension.
If you select an arc then select smart dimension the result will be the radius of the arc.
If you want the length of the arc you need to select one end point, then the next end point, then the arc itself, and finally select he location to place the dimension.
If you want the horizontal or the vertical dimension of the arc simply select the first then the second point of the arc then dimension location.
If you want the distance from a vertical line and tangent of the arc, select the vertical line hold the SHIFT key and select the circumference of the arc.
