Help: Creating Huge Maps

Hello, I am attempting to make a map based on a real world location that is about 12 square kilometers. I did get the map to be just under 1:1 to real life, but the problem is that in order to keep the dae file for the terrain small enough(it's still 400 MB when exported), I had to set the square size to 6.11. Due to this, terrain painting and deforming are really imprecise.
Should I just split the file into smaller chunks in Blender? Or is there a simpler solution to keeping the scale I'm looking for?

Image included for scale. That is a 5 lane road at one of the corners of the map.

Edit: I managed to solve the problem. The only decent resolution heightmap of the area was only 960x960. What I did was import the heightmap, scale it properly and smooth out the bumps, export to blender, then used blender to create a heightmap at the proper scale. The result is better than I could have hoped.