If you prefer to stay with deck view, then I suggest that you set the action fired to “row selected”, but you can place as action button at the right bottom corner of each line item for deck view.
How to trigger the action, it is upto your decision and at your convenience.
We are not able to resize the thumbnail of youtube recording which appears on the Appsheet detail view. In any case, app user need to click the screen on detail view once.
If you really want to set the youtube “autoplay” i.e. upon opening Detail view on Apphsheet, then you add parameter to YouTube URL.
Add this string to the end of your URL.
By doing this, Youtube recording will be started automatically upon opening detail view after prompted from deck view by action.
Try it out.