Hey kaizen,
Box Collision settings will only take effect if Is Dynamic Obstacle is enabled. Turn that on and you should see effects of your Offset and Extent once you either recast NavMesh or move mesh (if you have it set to recast when moving mesh).
There isn’t any documentation on this that I could find either. I will enter a report for our Documentation team to see if they can put something together for it.
Hope that helps! Let us know if you run into more questions with it.