320x50. This ad size is known as a Mobile Leaderboard, or Smartphone Leaderboard. It is most commonly used as an ad on mobile phones and listed as one of the top performers (JPG, GIF, PNG, HTML5).
Expansion not allowed for this unit. PARAMOUNT: Audio must be user-initiated. Default state is muted.
Regarding Nickelodeon and MTV, it only applies to the KCAs and MIAW microsites.