I have only printed a couple small panos, and I used Mpix.com, and Adoramapix. Both are of great quality, and reasonably priced. They both offer a standard sized print in a pano format, but they aren't very big. Mpix has a 5x15 print for $3.99, and Adorama has a 4x12 print for $0.99, and a 6x18 for $4.99. All of these prices are for their standard, matte finish.

If you want to go bigger, you can, but you will have to order one of their standard sized prints based on how wide you want the final image to be. This will give you a lot of empy space on the top and bottom of the print that will have to be trimmed away.

If you go this route, I would create a blank document the same size of the print you want, then past your final pano image into, as many times as you can fit on the document. You may be able to get 2 or three large panos, for the same price as a single larger print.