For a curvature based mesh, the element size is determined mathematically by the minimum number of elements that fit in a hypothetical circle.
Let the quarter circular arc be a part of the model. When the Min number of elements in a circle is set to 8, the completed imaginary circle contains 8 elements. So, the red arc contains 2 elements.
This option defines angle α (45 degrees in this example). The length h is computed as h~rα, where r is the radius of circle. For this option to have effect, hmin< h < hmax where hmin and hmax are sizes from Minimum element size and Maximum element size, respectively.