approved character backstory?

You'll want to make sure it falls into the lore of the world. If you are making up s new country or culture or major city, it might not get approved, or if you write yourself in as nobility (though noblility backstories do exist).

Length - I haven't heard anything either way

Open ended- yes and no? If a story teller decides to run something from your backstory, they can change your backstory to fit? I've seen people who had a wimpy scholar fiancé in the backstory and the person who showed up was the quinarian equivalent of Gaston. Basically, from what ive seen, storytellers will use what they like and change parts to fit their vision.