I use Allvideos on a site for gymnastic sports and the used joomla template is build as responsive.

In the documantation there is this info:

...where "width" and "height" are the media dimensions in pixels and "autoplay" is enabled using 1 (use 0 to disable it).

For example, a YouTube video with 600x450 dimensions that autoplays on page load would be displayed adding this plugin
tag in your Joomla! article (or K2 item, or Virtuemart product etc.): {youtube}he73js822|600|450|1{/youtube}

Is there a way you can use percentage for the media dimensions in AllVideos?

kind regars,
Ronald

No I am afraid not.