If you were a film photographer, and have no moved to digital photography, then you need to understand resolution.  I suspect resolution isn't something the film photographers of yesteryear had to understand.

A friend of mine spent $160 on a two A4-ish photos and a small 6 x 4 inch photo taken in a shopping centre.  I don't know how much photography costs, but if I'm paying that much for a photograph I expect the quality not to suffer from negligence.  One of the A4 photos appeared out of focus, but on closer inspection I was able to identify that it was blurry because it had been resampled from a lower resolution image.  I suspect the original photo may have been 3 megapixels or less.  This could only have happen if the photographer didn't understand the 'digital' aspect of photography.

So I'll try and put it into perspective for those photographers who don't have a grasp of how the resolution of digital photos relates to film photos.

I'm pretty sure the accepted resolution for printing a digital photo is 300 DPI (dots per inch) or more.  This means for ever square-inch there will be 9000 pixels (300 x 300).  So for a 6 x 4 inch photo (15 x 10 cm) your digital photo needs at least 2160000 pixels (2.1 megapixels).

So if we work backwards with a photo at 3:2 ratio, a 3.1 megapixel photo is should be printed no larger than 7.2 x 4.8 inches (18 x 12 cm) and a 5.1 megapixel photo is suitable for a photo 9.2 x 6.1 inches (23 x 15 cm).

People printing their own photos might be happy with the 200 DPI but a professional photographer should not be selling a photo at less than 300 DPI.  And resizing your photo to be larger is no different to lowering the print resolution, except the photo will look blurry instead of pixelated.

Understanding that a quality printed photo can only be achieved at 300 DPI also means that photographers who choose to publish their work online do not need to make the photos small to prevent art theft.

The photos that sifu submits on DevianArt that typical have a resolution of 680 x 450 pixels will only produce a photo about 2.3 x 1.5 inches (6 x 4 cm).  Even at 200 DPI the print size will only become 3.4 x 2.3 inches (9 x 6 cm).  So making the photos so small certainly does protect them, but you could make them a lot larger with any fear that people will be distributing high quality prints.

So, make sure your take photos with the maximum resolution and note that there is a limit to how large you can print a digital photo before it becomes blurry or pixelated.  And note that you can be pretty generous with the size of photos you put on the web without fear of it encouraging art theft.