After a long battle with photoshop and shorelines, I'm in the process of detailing a new terrain.
I do not use any custom textures/tiles for the terrain.
With that said and the terrain about 75% detailed, the file is already at 4mb with no bases placed on the terrain yet. Once the terrain is completely detailed and the bases and strat are placed on the terrain I would expect it to be over 6mb.
So, a 10mb terrain file isn't really that far fetched.