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Well, when you boot up NBT Forge you get 3 options: New Indev Map, Open Indev Map, Open Alpha Map. I had an Alpha map randomly generated for my server, so now I can just open it up and edit it with NBT Forge.
If you do not have your own server, and thus can not have one generated, try this one - freshly generated.
Zoomed all the way out, you see a light gray boarder around the map. I'd say that's the extent of the map. You wind up on the other end, if you were traveling it in-game. It's still really quite large.