Top Down View template question

I’m experimenting with the landscape mode and I’m using a heightmap created in Gaea.
I’ve tested the ‘1st person’ and ‘3rd person’ templates and it works fine. In the the ‘top down view’ template it needs the NavigationMeshBoundsVolume for the character to move on the heightmap, but my terrain is a couple of kms in size. Is it normal/safe to resize the NavigationMeshBoundsVolume to the height map’s? How does it translate in terms of performance?