How to correctly limit the content and strip HTML?

Question

First, I wouldn’t modify the string/word length of the content. Semantically, you’re dealing with an excerpt, so let’s focus on that.

Second, to limit the number of words returned for the excerpt, simply filter excerpt_length:

<?php
function wpse52673_filter_excerpt_length( $length ) {
    // Return (integer) value for
    // word-length of excerpt
    return 150;
}
add_filter( 'excerpt_length', 'wpse52673_filter_excerpt_length' );
?>

The added benefit of using the_excerpt() is that, for auto-generated excerpts, no HTML is parsed. So, if you don’t use user-defined excerpts, you’re done.

However, if you do use user-generated excerpts, but still want to strip HTML tags, simply remove any or all of the following filters that are applied to the_excerpt:

add_filter( 'the_excerpt',     'wptexturize'      );
add_filter( 'the_excerpt',     'convert_smilies'  );
add_filter( 'the_excerpt',     'convert_chars'    );
add_filter( 'the_excerpt',     'wpautop'          );
add_filter( 'the_excerpt',     'shortcode_unautop');

To remove one, simply call, e.g.:

remove_filter( 'the_excerpt', 'wptexturize' );