This mod creates a perimeter around your starting area. Currently, this can be either water or a wall.

The perimeter scales with your starting area with one exception - it will not shrink below the point where it could cut you off from your starting resource patches. As long as your starting area is at least 100%, you should see few if any biters within the starting area.

Your perimeter will not show on the map preview - to estimate its size, note how closely biters spawn (but note the caveat above). If you choose a very large starting area, it may be some time before you find the border.