I guess you know the only real difference between light woods and heavy woods is that heavy woods is impassible. Terrain that is impassable causes problems for the AI if there is too much of it. There may be ways around the problem, but it's going to be a lot of work.
My advice is to make the large area of light woods, scattering spots of light woods in the area you want to have woods, and possibly make some heavy woods in the center if you want the area impassable. It's not a great solution, but may work.
I wouldn't mind seeing the ability to make light woods of any size, with a large wooded area shown on the 2D map while the 3D engine figured out how to scatter patches of light woods in this area to give it the proper visual effect without actually exceeding the maximum number of light woods areas.
Hook