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.
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.