Setting up a CoH2 multiplayer map: video tutorial
The following image which is explained in the tutorial illustrates the different areas of a map and how their sizes are related.
Illustration of map areas and sizes
Should you create a map for team games, make sure to assign the player's to their respective team in "Scenarios->Teams", so they are not mixed between the different team sectors.
Should you have further questions regarding specific steps in the tutorial or regarding map sizes, don't hesitate to ask them in this thread.