a) long enough to proof, that you can handle RP
b) write so that you are the person in this story, depending on the story
c) Because we want only player, which are able to play this game right.
1. I think you can't say a concrete length that the story has to be, the story must give the readers the feeling of a roleplay, I think that's the most important point.
2. You have to write like you would be in the story (First person I think?)
3. Because, Illarion is an awesome game and so it needs only awesome players to hold this standard...
1) As long as you want as long it is good. but grammar is important too or they would think you are too stupid to RP.
2) Whatever point of view the GM's beginning part of the story is, (mine was third)
3) It is not hard, took me umm... thirty minutes. Only people with no imagination fail.
Try your best, if you don't suceed then admit defeat and don't bug the GM's.