Just to clarify, you can use any size you want, they're dynamically resizing them on the fly.
160x40/79ca2351235643339a9fb5e83fa3c2a4.jpg
Depending on what your design calls for, you can restrict either the height or the width.
For example, if you always want your images to be 600px wide, you can pass /600x9999/ and it will always restrict to the smaller value.
If you want your images to always be 600px tall, you can pass /9999x600/ and it will always return images that are 600px tall.
There is no cropping function, so passing both width and height doesn't do much.