Question: In my book I have a character that when you meet her she's already been through a lot and met a lot of people and just thought of the idea to write a book on her that showed her life before everything and leading up to the events in the actual book and I've seen that done a few times and I've noticed that it's not as long as an actual book it's always shorter so I'm not exactly sure how long I would makes it.
How long you make it depends entirely on the extent to which you develop her story.
In the examples you've seen, it may be that the author simply did not develop the character's backstory beyond a certain extent (needed for the first book), and then did not venture too far beyond than later when writing the prequel. It might also be that the author felt somewhat constrained by the facts that were already established about the character.
However, you have no reason to feel bound by such limitations. If you wanted to, you could create a very well-developed, full-length book about this character, of which only a small part addresses the issues in the other book. It's a matter of taking the time to find a story that really excites you, and developing it in exciting ways.
For that matter, you might create a story about this character that takes place years before the events that make her the person she is when she appears in the other story.
For example: A.C. Crispin wrote a trilogy of books about the life of Han Solo (the Star Wars character) before the events of the first film. In fact, there are over 90 Star Wars novels, including both prequels and sequels to the films.
The other lesson this teaches is that if your first work proves very popular, writing sequels or prequels is a great way to extend the franchise and increase your sales.
But that's down the road. The thing to focus on is making that first project successful.