Ok, I understand this.
I played some such huge map PBEMs with CMBO and during most turns nothing or very little happened because units were spread on such huge area. Based on my CMBN experiences 1.5km x 2km or maybe 2km x 2km would be still playable, especially if at least one side uses mostly motorized units. I tried playing one game using one of the master maps and my PC basically stopped. Got maybe 1fps. Just not enough power to handle maps that big.