Force the full size featured image to be used by the BP Media Extractor for `new_blog_post` activity types (See BP Trac ticket 8052)

bp-custom.php

<?php

// Exit if accessed directly.
if ( ! defined( 'ABSPATH' ) ) {
	exit;
}

/**
 * Force the full size featured image to be used by the BP Media Extractor for `new_blog_post` activity types.
 *
 * @see https://buddypress.trac.wordpress.org/ticket/8052 for the context.
 * @see https://codex.buddypress.org/themes/bp-custom-php/ to learn how to add the `bp-custom.php` file into your WP site.
 */
function teeboy4real_new_blog_post_extractor_args( $args = array(), $content = '', $activity = array() ) {
	if ( isset( $activity['type'] ) && 'new_blog_post' === $activity['type'] ) {
		unset( $args['width'] );
	}

	return $args;
}
add_filter( 'bp_activity_create_summary_extractor_args', 'teeboy4real_new_blog_post_extractor_args', 10, 3 );

/*
 * You'll also need to add a new css rule to your theme's styles:
 * `.activity-list .activity-item.new_blog_post .activity-inner img { max-width: 100%; }`
 */