Reducing the size of a JPEG file or photo

Are you trying to save storage by reducing the file size of photos stored in the Photo library? That is not possible. The Photos.app is using a lossless workflow. When we edit a photo it may even need more storage, because Photos is always storing the original plus the edited version.

To change the original file in Photos you have to create a smaller version by reducing the size on export, then reimport the smaller version to Photos and remove the larger version. I am always keeping the full resolution versions however. We may regret to have lost the full resolution versions one day, when we want to make a high quality print or view the photo on a larger display.

What size specifically?

For what purpose?

The file size? As in how many megabytes it takes up, or the physical dimensions a.k.a resolution of the Photo?

If using the Photos app, you can open the Photo, then go to there File Menu ➜ Export ➜ Export 1 Photo

And then modify the JPEG quality and the Size.

Quality will reduce the overall file size and the image will be reduced in quality i.e compressed, but retain its current dimensions.

Size will change the photo's dimensions and by extension also the resulting file size but does not compress the file.

OK... what have you tried?

You can open the image in preview and 'save as'. You can modify the quality options in the save as dialog to reduce the image size.