<p>When applying the Single Responsibility Principle and looking at a class's reason to change, how do you determine whether that reason too change is too granular, or not granular enough?</p>

Rewrite your code in a passing style!
Seriously, an old code base will have hundreds of errors - but that's why we have novice/intern programmers. Correcting FxCop violations is a great way to get an overview of the code base and also learn how to write conforming .NET code.
So just bite the bullet, drink lots of caffeine, and just get through it in a couple days!