Deleting files for freeing the disk space
Honestly delete nothing in those directories manually. You'll likely crash your system if it even lets you delete them. Stick with files in "userland." which would be at the path /Users/[YOURUSERNAME]/."

If you are running out of disk space (which is completely different from running out of memory) I would use a gui tool like Grand Perspective and then delete the files that you no longer need that occupy locations in your user directory.

I'm sure there are shell commands to list, recursively, files in specific directories in order of file size, but I don't know how to do that, preferring GUI tools that do the same thing.

If you are running out of memory in your MBA then the only choice you have (as all MBA memory is non-upgradable) is to see if you have a bunch of things running at startup (system preferences > users > [YOUR USERNAME] > login items) and remove some of them or just run fewer apps at the same time.