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Let's start with an image. This one is, as you can see, 2882x1921 pixels. [Editor's Note: Click images to see larger versions.] It happens to be set at 240 dpi, but that is irrelevant.
Now select the marquee tool. In the tool options choose "Fixed Aspect Ratio." Say we want to target an 8x10 print; we then choose an aspect ratio of 5 wide by 4 tall (could just as well be 10 x 8, 20 x 16, or whatever multiple you like best).
Now select the crop you want. The marquee will constrain you to choose something that will fit in the 8x10 print.
Once you are satisfied with your selection, go to the Image menu and choose "Crop."
This is your cropped image, and this is how your 8x10 print will look like.
Lastly, you need to resize it for printing. Go to the image menu and choose image size. Uncheck the "Resample image box." Then, in the document size portion of the dialog, choose inches as your unit of measurement. Set the width to 10 inches; the height will automatically change to 8. The dpi will also change to reflect the pixel density. In this case, the resolution ended up being nearly 230 dpi, which is plenty good for printing.
Say you had attempted to print your image as a humongous 40x32 poster. In that case, the resolution would have dropped to 55 dpi, which would have looked pretty awful when observed at close distance.
If you use 120 dpi as a safe boundary, you can print your 6 megapixel images up to 20x30, provided they are in focus and properly sharpened.
From this thread: /forum/remark,14034484