This is a step-by-step instruction with illustration for you.
Step 1: Open Finder and navigate to your image folder
Step 3: Inside Preview, choose Tools\Adjust size...
Step 4: Type in your width and height, click the lock to unlock the ratio if you want to change the ratio of your picture. Then click OK
Step 5: Click File\ Save a version to save your file. Remember if you want to keep the original file (as usual), please choose Duplicate in the next warning of Preview
|The warning from system that you might duplicate the file to keep the original.|
If you have any questions, please leave comment below, I'll try to help.