The mega airfield with the spread out town is called laf4x4m. It is a 4 mile square land object. For the object terrain only (just the ground, not including the non-terrainset buildings, gun pits, hangar, etc), the poly count is 28,440. The vertex count it 21,400.
So to make sure you create a 4 mile land object that won't tax frame rate, it's a safe bet to make sure you come in under those numbers. It helps even more when you don't have to add all the town buildings, gun pits, hangars, etc.
Blender's decimate tool is a huge time saver. I went from 4 million polys to 21,000 polys in a matter of minutes. The next step is to section off the whole thing into a structured hierarchy, which AC3D does really well and really quickly. You basically create a cookie cutter object, move it over the land object, and slice up the land object into discrete 1/4 mile quadrants, and order those quadrants in the hierarchy in a structured pattern. That step cuts down on the collision model calculations, and leaves you with a flyable terrain.