Overwrite Yoast SEO Facebook & Twitter sharing images declaration with a custom one.

yoast-seo-social-images-sizes.php

<?php
/**
 * Std image size for social sharing.
 **/
add_action( 'after_setup_theme', 'mytheme_setup_theme' );
function mytheme_setup_theme() {
	add_image_size( 'social-800x420', 800, 420, true );
}

/**
 * Replace default sizes with std one.
 **/
add_filter( 'wpseo_opengraph_image_size', 'mytheme_social_image_size' );
add_filter( 'wpseo_twitter_image_size', 'mytheme_social_image_size' );
function mytheme_social_image_size($val) {
	return 'social-800x420';
}