To be honest there is many different ways to resize several images at once on your Mac. There is a lot of Mac apps that do such thing like an Automator workflow or download one of the many apps available to do that job.
STEP 1: Select all the images you want to resize.
STEP 2: Right click on them and select “Open With Preview.â€
STEP 3: In Preview, go to Edit > Select All.
STEP 4: Once all images are selected, go to Tools > Adjust Size.
STEP 5: Enter the width or height value to resize your photos to your liking. If you’re only going to resize the width for example, make sure “Scale Proportionally†is selected. This will ensure that your images are proportionally resized.
STEP 6: Save and close.