From an article in EOS Magazine:
"What is the largest size print you can make from your image file? Do a search on the internet and there is no shortage of information. There are charts and tables, mathematical calculations and countless opinions. What is rare, however, is a photographer who has actually tested this.
One such is David Pogue, an American technology writer and television presenter. For his show "It's all Geek to me", he made three 16 X 24 inch photo prints. One was from a 13 megapixel file. He also made prints from the same image, but downsized to 8 megapixels and 6 megapixels. The prints were hung on a wall in Times Square, New York, and passers-by asked if they could see the difference, the rest made guesses and were wrong. Only one person correctly ranked the prints in megapixel order -- and she turned out to be a professor of photography.
There is the temptation to shoot at your camera's highest resolution. We all do it, just in case we capture the perfect image and want it enlarged to the size of a wall. However, keep in mind that all EOS cameras allow you to shoot at less than their maximum quality setting. On the EOS 650D, for example, the 'Large/Fine' and "RAW" settings give 18 megapixels, but other settings will record 8, 4.5, 2.5, or 0.35 megapixels. You could save computer storage space selecting an appropriate setting."