Hi Ebwlx,
Your contact definition looks fine to me. The two contact regions do not need to have same shape or size.
As for the slider & value under Auto Detection, Workbench Mechanical generates contact pairs between surfaces based on their proximity (tolerance) to one another. You can change the tolerance using the slider, or alternatively (suggested approach) set the Tolerance Type to "Value" and directly input your desired tolerance value.
You are navigating away from the AIS Discovery experience