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.