1. Open the image you want to resize, using "Photoshop". You can do this once you open Photoshop and then in the top menu select "File" and then "Open" or you can just press "CMD"+"O".

2. From top menu select "Image" abd then "Image size". Alternatively you can click "CMD"+"ALT"+"I".

3. Change the size from "cm" to "pixels" and input the height and/or width to resize the image to the size required.

4. Press "CMD"+"S" or "File" and "Save" or "Save as..." from the top menu to save the image in the desired size.

That's it!