Limit video upload size per user role?

Drupal and CDN2 don't have any interaction beyond a shared video token -- as a result, the backend service has no knowledge of what users exist on a particular drupal site. So it's technically not feasible to do so.

You can, however, trap the callback that CDN2 provides and look at the assets that are returned; if they're over a certain size you can reject the video and prevent it from being published. This wouldn't prevent the video from being transcoded, but it would allow you some control over what is downloaded.