The default is OK for Cities: Skylines, but reading elsewhere, a setting of 24 or 25 brings the horizontal scale nearer to 'real' real Earth distances.
The vertical scaling eludes me because after following BloodyPenguin's excellent tutorial and putting my local Terrain Party map through Wilbur and then importing into the CSL map editor, I have what are 100m hills on the outskirts of town rearing up to approx 400m...