I am starting development for my first 3D game and a lot of the game play will take place on a large map that shows the where all of your units are and your geographical surroundings. What software should I use to design this map?
Are you looking for a program to draw a world map with? Or a program to make a tilemap (I know in your last thread you said it would be like the Civilization series).
If it is the first one I would probably draw it by hand then touch it up on the computer which you can then use in different scales to show the world map and even the mini map.
If you are looking for tile map editor you can either make your own or use one that is available already (Though you will have to make something in your framework to load the files that editor generates).
A clean and simple one I like using is "Tiled" http://www.mapeditor.org/ . It supports isometric tile maps and hexagon tiles like civilization uses also. You will just need to make the tile sets for it and something to get the information from the TMX files into your game.