set-posts-content-length.php

<?php
/**
 * Set Limit of content length.
 *
 * @param  mixed $content
 * @return void
 */
function wbcom_limit_content_length( $content ) {
	if ( 'post' === get_post_type() ) {
		return mb_strimwidth( strip_tags( $content ), 0, 100, '...' );
	} else {
		return $content;
	}
}
add_filter( 'the_content', 'wbcom_limit_content_length' );