Hm... For the most part I just have ideas in my head and don't run with notes, since when I do make notes they are really just something in the world replicated as "notes". An example is my magic system, I have rules, balance, and how the characters use them; The rest I just have some tid bits on characters personalities and how they think of certain characters, all in my head. Well... I do have the smaller notes on characters, but those are more like a garbled mess of facts about the character.
Let's suppose I made a character note, it would often go like this:
Name.
Age.
Gender.
Likes, dislikes, and demeanor.
Thoughts on close friend and/or rival.
And most of it can fit on a small piece of note book paper no bigger than my palm. So I guess it depends, either it is detailed (most often doing so if it is something that needs to be understood in depth and can have superficial aspects when implemented in the actual story), or very loose (most often when it needs to be dynamic and shouldn't stay still forever.)
