Responsive Embedded Youtube Video

I'd like to make the YouTube Video embedded in Attachments responsive and with a maximum size limit.

I'm not a coder, so I need example code for the CSS and any modification to the YouTube embed code.

I would greatly appreciate any help you can give.

Lance

Would you clarify what you mean by "in Attachments"?

You can style the iframe with the following CSS and it will take the parent container width and will have responsive width as per the parent width

iframe {
  display: block;
  width: 100%;
}

I'm using the Nop Attachments plugin and need to understand how to make the embedded Youtube videos responsive.