Most child themes include code which determines the default embed sizes for videos and other embedded media.
You can also change these values or add your own which effect all media embeds globally.
Here’s an example of what you’ll find in all StudioPress child themes:
$content_width = apply_filters( 'content_width', 740, 740, 1140 );
And here’s some code you can add to other child themes:
if ( ! isset( $content_width ) ) $content_width = 600;
You can also add a height to this code:
if ( ! isset( $content_width ) ) $content_width = 600; $content_height = 300;
But what if you only want to change the embed size on one specific page or post? You can also do this using a custom function which includes a conditional tag.