GitHub REST API search documentation

I have a question regarding the repository search API (I hope this is the right place, otherwise please tell me where I can ask about this): Search - GitHub Docs
There are lots of variables returned but I can’t find a description of these variables. I’m especially interested in the size variable. I found a few repos that have a size of 0 but they had files so this doesn’t refer to file size. Can I read up on what the variables are somewhere? I’m also interested in the other variables.