I've got an option for a plugin im using to change the line to localhost but have no idea where it would go and how much data it would eat up.
I'd like to store all this data locally since Im not interested in paying for something like cloud sql

Any pointers?

This is for a self hosted server

What do you mean that you are unsure where it would go and how much data it would eat up?

If I self host does it go somewhere specific or do I need to specify?
And what kind of disk space and bandwidth am I looking at

All of this entirely depends on what you want to do with it.

Google how to set the directory for MySQL, it's straightforward.

As for space: I've hosted local databases with sizes ranging from 5MB to 350GB. We can't tell you how big it will be if you can't tell us what you'll be doing.

What is this a plugin for and what does it do?