In general, the crack contour refers to the concentric toroidal volume of 3D elements situated around the crack front nodes. These elements are used to perform the fracture parameter calculations at each node on the crack front (for further information, see the MAPDL Fracture Analsyis Guide 22.214.171.124. Domain Integral Method for Calculating the Fracture Parameters)
In the pic below, you can see 6 concenctric rings, which represent 6 contours (the number of which the user can specify in the semi-elliptical crack settings). The largest contour radius is the length from the center crack front node to the boundary of the last specified contour.
In choosing the radius size, overall you want to make sure that it is of sufficient size to capture the stress gradient around the crack tip. You can also base this radius on the approximate size of your initial crack length. If you are using a tet mesh for the Mesh Method, you will have the option to specify the Front Element Size. If your crack length is 'a', it is recommended to set that size to a/8. You can also experiment with different mesh refinements to see how the results compare. Make sure that the results for contours 3-6 converge. Usually the first two contours will suffer from a lack of accuracy since they are closest to the crack tip stress field.
In the past, a structured hex mesh was required to obtain good accuracy for the fracture parameter calculations. However, using a tet mesh will produce comparable results, especially in light of the Unstructured Mesh Method (UMM) which ANSYS software automatically employs when a tet mesh is present. Note that if you want to perform SMART crack growth a hex mesh cannot be used and you'll need to specify a tet mesh for the Mesh Method option.