This may not be the main reason for your question, but if you wanted to resize an image for use elsewhere (e.g. for use on a web page), you can choose to resize it upon export.
- Within Photos, select your image(s)
- Press the shiftcommandE keys (or navigate to File > Export > Export Photos...)
- In the pop-up window that appears, click on the down arrow to the right of the Photo Kind option (see below)
- This reveals the Quality, Color Profile and Size options
- Click on the Size drop-down menu and select one of the preset options or choose the Custom option
- OPTIONAL: - If you selected the Custom option at Step 5 you can now specify dimensions for width or height to resize the image
- Click on the blue Export button
- Select the location you want the photos exported to
- Click on the Export button
Now your photo(s) have been exported with the size you chose to the location you chose and these are available to other applications (and within the Finder).
Using 3rd party apps as helpers within Photos
Depending on the version of macOS (and Photos) you have installed, you can also use 3rd party apps as helper apps to edit photos. You can launch these from within Photos, make your edits, and then upon saving the changes appear automatically within the Photos app. This includes scaling changes.
To do this:
- Right-click (or control-click) on the photo you want to edit
- Select the Edit With option
- Select the 3rd party app (e.g. Photoshop etc)
- The photo you clicked on at Step 1 opens in the 3rd party
- Make any editing changes you require (including resizing/scaling the photo)
- Save the changes within the app.
Now your edited photos appear with all changes intact within the Photos app.
NOTE: The app you select at Step 3 must already be installed on your Mac for it to be available.