How to resize an image on a Mac

Preview

1. Open the image and choose Tools → Adjust Size.
2. Enter a width or height, keep Scale proportionally on, and click OK.
3. Save, or File → Export to keep the original.

Finder: several at once

Select images, right-click, choose Quick Actions → Convert Image, and pick Image Size: Small, Medium or Large.

Terminal

sips -Z 1600 photo.jpg

That shrinks the longest side to 1600 pixels, in place. Copy the file first if you want to keep the original.

Smaller file, same size

To cut the file size without changing the dimensions, save as JPEG at a lower quality: in Preview’s File → Export, drag Quality down. NotchMind, which I make, converts to JPEG or HEIC at the quality you set (92% by default) on its Converter page. See converter settings.