The Epic map is 18x18, while Gigantic is 16x16--so yes, Epic is larger.
My computer has trouble with Gigantic when there's too many races and habitable planets.
I've tried to make custom maps, but I can't seem to do it in Notepad and I don't know what else I could use. I can design them okay, but they always crash the program when i try to load them. And, since I can't start threads due to the new forum bug, I can't really ask about it in the depth I want.
Edit: The maps are easy to locate, being in a suitably named folder, and if you open them with notepad they show up as XML files. They're pretty simple--it's basically X item at Y coordinate. So, you can put a planet of type such-and-such at a certain grid location, or a star of type such-and-such at another grid location, or a resource of type 1, 2, 3, 4, 5 at such-and-such location... easy. But apparently, Notepad doesn't save the file properly, because the game dies when I choose the map from the menu, even if it's just a renamed version of one of the standard maps.