Let’s face it, if you’re a developer you want to make things as simple as possible for your clients. One way to do this is to set the default media sizes when your theme is activated. Your users should never need to try to figure out the width of the content area. They shouldn’t have find where the default settings are located. They shouldn’t have to worry about any of that. When they upload an image or use an embed code, the correct sizes should already be set. Here’s how you can do that for them.

// Reset default media sizes
if ( is_admin() && isset( $_GET['activated'] ) && $pagenow == 'themes.php' ){
	global $wpdb;
	$wpdb->query("UPDATE $wpdb->options SET option_value='200' WHERE option_name='thumbnail_size_w'");
	$wpdb->query("UPDATE $wpdb->options SET option_value='200' WHERE option_name='thumbnail_size_h'");
	$wpdb->query("UPDATE $wpdb->options SET option_value='1' WHERE option_name='thumbnail_crop'");
	
	$wpdb->query("UPDATE $wpdb->options SET option_value='400' WHERE option_name='medium_size_w'");
	$wpdb->query("UPDATE $wpdb->options SET option_value='400' WHERE option_name='medium_size_h'");
	
	$wpdb->query("UPDATE $wpdb->options SET option_value='800' WHERE option_name='large_size_w'");
	$wpdb->query("UPDATE $wpdb->options SET option_value='800' WHERE option_name='large_size_h'");

	$wpdb->query("UPDATE $wpdb->options SET option_value='800' WHERE option_name='embed_size_w'");
	$wpdb->query("UPDATE $wpdb->options SET option_value='0' WHERE option_name='embed_size_h'");
	$wpdb->query("UPDATE $wpdb->options SET option_value='1' WHERE option_name='embed_autourls'");
}

This piece of code goes in the functions.php file of the theme. When the theme is activated, it sets all the default media sizes available on the Settings > Media page. Feel free to remove any lines that you don’t need. Then change the remaining values to fit the size of your theme and you’re ready to go!