It´s not a matter of size in square miles or Km, , but of desntity of objects, roads, airbases, etc... There is a physical limit to how many objects and data you can place in a map and still be playeable...
You migh get the whole of Afghanistan in a map but it would require 128 Gb of Ram to run it making it inpractical to run.. much less sell... or just crahing DCS when you load it...