By clicking the Zoom button, you can zoom in or out on the Civ map. Simply enter a number as the zoom percentage, such as 75 for 75% zoom, and you'll be at that zoom level! You can continue to edit the map as you would at 100% zoom.
The trade-off is that the map is drawn more slowly the further zoomed out you are. It's generally decent at 50%, but while 25% works fine for seeing the map, I'd recommend zooming in a bit further for editing purposes. Note that you can optimize this in favor of faster speed or higher quality in settings.