how do you make the map's size bigger?

In the upper right corner of ZE, under "Advanced", click on Terrain.
Then, click on "Create New Terrain"
Enter in your desired terrain size, just make sure that both values are a power of 2 (ex: 128x128, 256x256, 512x512, 1024x1024, etc.)

But do so before adding anything to the map or changing terrain heights, doing so afterwards will move things all around objects, land features etc.

I've had too increase the size after doing some work, and it doesn't move objects! Well, it keeps the same objects in the same relation to each other. (if you have a town, for example, and you change the size, it won't mess up the layout of the town.)

But yes, any terrain or height will be erased.

I never knew this... I'll have try it. Thanks!