1. Find the photos you want to resize, in Finder.
2. Select the photo (or photos) you want to resize, then right-click and select Open With > Preview.
3. Click Tools > Adjust Size....
4. Adjust the settings to the size needed. The default measurement is in inches, but it usually makes more sense to change this to either pixels or percent.
5. Make sure the Scale proportionally box is checked unless you want to stretch or squish your photo.
6. The Resample image box, if checked, will allow you to make the file size smaller by adjusting the Resolution number.
You can also use Preview to rotate or flip an image. For more information, please see Apple's KB article here: https://support.apple.com/guide/preview/resize-rotate-or-flip-an-image-prvw2015/mac