What Believable3D said.

It's really simple, we need two pieces of information:

a) the physical dimensions of the printed output
b) the number dots / pixels per inch required to produce good quality results

Let's say your print size is 12 inches x 16 inches: if your print shop wants the image at 300 dpi, you will need to make the image (12 x 300) x (16 x 300) pixels. That's 3600 x 4800 pixels.

There are a lot of reasons why you may not need the image to have as many as 300 dpi, but printers and designers are lazy and 300 dpi is safe.

If you want a rambling, long-winded discussion about any of this click on the top link in my sig.

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