Adobe Photoshop PSD images, like other bitmap images, lose sharpness when they're resized. It's impossible to change the size of a PSD while losing absolutely no definition, but the loss can be minimized by using the right Photoshop settings. By resampling the image -- changing the number of pixels it contains -- according to special settings optimized for size modifications, you can greatly improve the definition of your resized images.
Open the PSD image in Photoshop.
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Click "Image," then "Image Size." The Image Size dialog will appear.
Make sure the "Resample Image" box is checked, and choose a technique from the drop-down menu. If you're shrinking the image, select "Bicubic Sharper"; if you're enlarging it, select "Bicubic Smoother."
Enter the new dimensions for the image in the boxes above, and click "OK."
Press Ctrl-Z to undo the resizing operation if the image didn't end up the size you want, then repeat the above steps to try a new size. Always undo the operation before performing another one -- resizing an image multiple times will gradually sap its sharpness.