Colin Phillips (Alumni)
This document available from Microsoft is a great reference point to know what each service application, content DB, system DB, and every other SharePoint 2013 DB is all about, and the estimated sizes each one can grow to. The document is very large (over a meter tall – that’s over a yard for you American folks), and nearly as equally wide.
http://www.microsoft.com/en-us/download/details.aspx?id=30363
The link comes in the form of either a PDF or a Visio VSD file, so you can take your pick as to your preferred format.