The first step is to make sure your image is in RGB color mode @ 8 Bits/Channel. Check example (1a) below to see if your file matches these settings. If not, change them.
(1a)
Now open the image size dialog as highlighted in (1b) below.
(1b)
(1c)
As highlighted in example (1c), set the document width and height to the exact size you want your image to be in inches.
Next, set the resolution according to the size print you are ordering.
For prints smaller than 16x20 inches, the resolution should be 305 pixels/inch. For prints 16x20 and larger, set the resolution to 152.5 pixels/inch.
(1a)
(1b)
(1c)
