As pointed out by LifeHacker, the following command will do this very easily:
sips -Z 640 *.jpg
To quote their explanation:
“sips is the command being used and -Z tells it to maintain the image’s aspect ratio. “640” is the maximum height and width to be used and “*.jpg” instructs your computer to downsize every image ending in .jpg. It’s really simple and shrinks your images very quickly. Be sure to make a copy first if you want to preserve their larger size as well.”
Source: http://lifehacker.com/5962420/batch-resize-images-quickly-in-the-os-x-terminal