Hi earbash,
You can set property ResizeMode="AutoAdjust", then editor will auto adjust the height.
Another way is use the external css to adjust the youtube thumbnail size in edit mode.
1. set editor.EditorWysiwygModeCss="example.css"
2. add the code below into the example.css
iframe
{
width: 560px;
height:315px;
}
Regards,
Ken