how do I find the largest files on my MacBook air?

Here is how I find large files on my Mac.

• Pick some size as the minimum size you are looking for. I personally choose 500MB; I will find all files 500MB or larger.
• Open Finder.
• Use Finder's menu to choose File > Find
• Change "Kind" to "File Size" in the search criterion that appears.
• Complete that row: "File Size" "Is Greater Than" "500" "MB"

The following has instructions on how to find large files: OS X El Capitan: Increase disk space

Another option is OmniDiskSweeper -- I haven't used it and can't personally recommend it: https://www.omnigroup.com/more

There is no need for you to defrag -- your Mac has built in automatic maintenance routines.