The main sources of information are always linked to in the first two posts of this thread, and the official Rules and Guidelines for this particular game.
It is recommended to not post anything shorter than 1:30min (unless otherwise stated), and ideally not exceed 10min. A good "ballpark" for maximum song length is 6-7 minutes. But it's not a "you're out" situation if your song is 10 minutes long.
If I should make that more clear in the rule set, please let me know
Only relevant for the Mix(ing) Challenge. The Songwriting Competition can be a MP3 or SoundCloud Link only. As long as the material is downloadable.
But if you want to show off your high quality material, then provide a WAV or FLAC (if you're low on bandwidth) as well. (personally, I do prefer WAV, as this doesn't introduce unwanted sound degradation through a 3rd party CODEC where you have no influence on the sound, see SoundCloud)