public float minRegionArea { get; set; }

This property allows you to cull away small non-connected NavMesh regions. NavMesh regions whose surface area is smaller than the specified value, will be removed.

Note: some regions may not get removed. The NavMesh is built in parallel as a grid of tiles. If a region straddles a tile boundary, the region is not removed. The reason for this is that the region pruning happens at a stage in the build process where surrounding tiles are not available.