Preview is one of the best parts of macOS. It's an image viewer, lightweight PDF editor and more. Six Colours reminds us that it's also probably the simplest way to quickly batch resize a bunch of images.
Just open up all the image you want in Preview, select them all from the sidebar, then click Tools > Adjust Size. Set your width or height, and all the images you selected will instantly get resized.
Quick Tip: Batch resize images in Preview [Six Colours]