[quote=@Rare]
Yeah, you never did say how long it needs to be...
[/quote]

That's deliberate. It doesn't need to be any kind of long or short; whatever's natural is fine. The voters are in charge of deciding how long is too long, or how short is too short, for them, personally.

[quote=@XxLyraxX]
May the song we pick be a fan-made one?
[/quote]

Anything you like! This, too, is really up to the voters. If people hate the song, that might hurt your odds of winning -- but then again it's not a song-picking contest, it's a writing contest, so you've really got as much leeway as you dare to take. The ball is totally in your court.