I am by no means any photographer who generates thousands of pictures per year and has apps and workflows around all that, but a need to batch resize big pictures occurs occasionally in my existence. Since I am a good friend with console, I tend to use a simple command or a script instead of downloading more and more apps and clogging my system. This is what I have been using lately:
sips -Z 2560 *.jpg
This simply resizes all jpg files in current directory to maximum width and
height of 2560px, while keeping the image aspect ratio (the
-Z part takes care of that).