Mhh... the HRE of which time period exactly?
Here's a handy "timeline"-gif: http://commons.wikimedia.org/wiki/File:HRR.gif
Have you decided on internal divisions yet? Like Imperial Circles, something more abstract, say along geographical/cultural sub-regions or (lol) actual dynastic/political borders? Because, you know, especially the latter would be *one hell* of a task... ( http://upload.wikimedia.org/wikipedia/commons/d/d1/Holy_Roman_Empire_1648_blank.png