I made a print 4 ft. wide of a panorama image shot on the X3 cam. It was in total about 9,500 px wide, and it looks pretty good from a normal viewing distance. Obviously getting close to the image is going to reveal its flaws but overall it was pretty **** good.
Since the X5 blows the X3 out of the water for photo quality, I would say on a standard 4:3 aspect you could print about 20" wide and it would still look good from a normal viewing distance.
I have a new pano to print that is 21,000px wide, thinking of doing a 6 ft. wide print... If only framing it didn't cost a small fortune...