The idea for a feature is like the name of character. You then need to develop a backstory (figure out how it works), then for the product (novel) to work, you need to weave that character in with the other characters (features).
This gets harder over time.
A good novel doesn’t have so many characters that you can’t keep them straight in your head, likewise a good product doesn’t have so many features that you are constantly surprised.
Dunbar’s number (between 100 and 230) describes how many personal relationships a person can maintain. This is relevant when thinking about the number of features a piece of software can have before becoming overwhelming. http://en.wikipedia.org/wiki/Dunbar’s_number
That requires defining the minimum feature, kind of like you might only tangentially know the maintenance worker at work, the login feature is small, where as sending a tweet is like the CEO for twitter…. rambling.