If your talking about going into the code - then it seems you are referring to using embed code for the youtube video component - rather than click and drag - as their is no way to 'go into the code'.
I am using the youtube component of marketo - not just a youtube include code.
I did use embed code for a form on a different landing page; however, using embed code renders the reporting useless e.g. sign ups, etc. I assume this would be the same for youtube video component.
If the above is true - and embed code is not optimal - how can i set the width, height, class for a youtube video component - of marketo?