This is what was said in another thread:
Max zoom out:
Since our maps vary greatly in size I`ll have to describe it myself by giving a few examples from standard skirmish gameplay.
Map with two player towns on it would be around 6 by 6 max zoomed out screens.
Map with four player towns - 9 by 9.
With 6 player towns - 11 by 11.
With 8 - 13 by 13.