Which image are you using? The 4th ed Eberron maps are ususally pretty large, they might be the best to scale up. Here's the
Khorvaire map if you don't already have it.
As for programs, I've had some success with this online processor, but I've never tried enlarging things to that scale. Hmm, didn't seem to work for me at all using the image I linked above.
Edit: I tried a free program called
SmillaEnlarger, and it seemed to work well. Not sure what type of quality you're looking for though. I enlarged
this map of Khorvaire to 7471 pixels in width, which is approximately 99 inches. If you use the program, make sure you use the sharpen option. It works wonders.