Resize or convert images in bulk with sips — via SS64

The easy way

1. In Terminal, cd into the folder with your images, then run the command. It creates a new small folder (and stops if one already exists, so nothing gets overwritten) and saves resized copies there, with neither side bigger than 1600 pixels.
2. Convert formats to a new file name that doesn't exist yet: sips -s format jpeg photo.heic --out photo-converted.jpg (formats include jpeg, png, tiff, gif, bmp).
3. Check an image's details: sips -g pixelWidth -g pixelHeight photo.jpg.

Command

Run in Terminal (Applications › Utilities).

mkdir small && sips -Z 1600 ./*.jpg --out small

Always use --out with a new folder or an unused file name so your originals and other files are never overwritten. To undo, delete only the files and folder the command created.