Change Yoast SEO default image size

functions.php

<?php

function theme_yoastseo_opengraph_image_size() {
	return 'large';
}

add_filter( 'wpseo_opengraph_image_size', 'theme_yoastseo_opengraph_image_size', 10 );

function theme_yoastseo_twitter_image_size() {
	return 'large';
}

add_filter( 'wpseo_twitter_image_size', 'theme_yoastseo_twitter_image_size', 10 );