For many years, Kodak had a display in Grand Central Station in New York that was called the "Colorama". This was a 18x60 FOOT backlit transparency. For most of the 40 years this was in place, the images were created using a larger format original, but there was a classic instance in the 1970's when Kodak bragged that they had used a 35mm Kodachrome transparency to create the image.

That means that they enlarged a 1 x1.5 inch film chip to create an 18x40 foot display transparency - an enlargement of over 200X (that's 20,000%).

As Denis points out, a critical factor that governs how much an image can be enlarged is what is the viewing distance. In the case of the Colorama, the display transparency was hung high over the main hall of Grand Central Station, so viewers were 60-80 (and that's just my guess) feet away. If they had been closer, they likely would have been able to see some image deterioration, but at that viewing distance, the granularity in the image was smoothed out to an acceptable degree.