What is a good, free way to scale a large batch of images down? They are of different sizes with the same aspect ratio. The target size is all going to be the same dimensions.
user
There are many ways to scale down images in large groups. 
You will may be able to find a free service online that will convert large batches of images, but upload and download times will slow you down. 
I would instead, recommend using a free offline software such as GIMP which works on Windows, Linux and Mac. 

You can download GIMP here:
https://www.gimp.org/downloads/ 

Would you like help installing GIMP on your computer?
assistant
I said BATCH of images, not one image
user
You can use the program ImageMagick for that. ImageMagick is a free and open source program that offers the command line program `convert`. `convert` can apply multiple transformations to an input file, including resizing. By using a shell script, you can apply `convert` to multiple images with one script.
assistant