Is there a way to resize a bmp map i created with Ascension's tutorial to something larger without pixellating it?
It's currently 2000x2000, and is about 6 inches across. I'd like it to be about 4 times that size (about 20 inches - to print at small poster size and put on the wall), but i don't want my coasts and everything to turn all jaggy.
First, I'd like to point you to Redrobes's great tutorial on bitmapped images and resolution: [Award Winner] Bitmapped Images - The technical side of things explained.
In answer to your question:
With a 2000x2000 image, your approximate 6 inches wide is likely at 300dpi (dots per inch). For something on the wall, you probably don't want to go as low as 100dpi (which is what 2000x2000 would be at 20 inches). Unfortunately the only way to increase the resolution is to add more pixels, and if you do so by scaling it, it's going to pixellate (due to the scaling mechanisms used by a computer, explained in the tutorial linked above).
The short answer is, your best bet is to redraw the image at 4000x4000 pixels at least (which would give you 200dpi in your 20-inch map). It's a pain, but it's the only way you'll get the same quality at a larger size.
It depends on the techniques you used in the original. Some upsizing is easily unpixellated, and some is not. I deal with low-res graphics at my day job, so I have some techniques.
One technique that I use a lot, I call "melt." It consists simply of blurring and then sharpening an image. The final effect is to smooth off those rough, pixellated edges. Unfortunately, it also smooths corners & sharp points, which you might need to repair afterward.
Adding noise can also help a lot in making pixellation less noticeable. (It mitigates JPG artifacts, too.)
There is also a fractal upsizer that you might look into. You might find a demo. Sorry, I can't recall the name.
If you get stuck then post a small swatch / sample of the map and we can see how it would look upscaled.