Save an image in a different size in Photoshop
1. Go to “File” then “Export” and click on “Save for Web”
2. Select the format you want to export
3. Change the image size down below
4. By clicking on the symbol next to it you can disable the original proportions if you want
5. Press “Save…” to save it.
If you choose to save your image as “JPEG” than keep in mind that you can adjust the compression quality. If you are not sure which format you should take, then I would recommend using PNG-24 because there you don’t have to make any additional changes.
Do not use this technique to save images for printing!