Way to show content of a post, but if exceeds character limit revert to excerpt?

replace the_content(); with echo wpse_limit_content();

   function wpse_limit_content() {
   $content = $post->post_content;
   $MAX_LENGTH = 100;

    if ( strlen(  $content )  <= $MAX_LENGTH )
        return apply_filters('the_content', $content );

    $s2  = substr( $content, 0, $MAX_LENGTH  );
    $s3  = preg_split( "/\s+(?=\S*+$)/", $s2 );
    $s4  = $s3[0];

        return apply_filters( 'the_excerpt', $s4 );

  }

If the string is to long it trims it to give you a nice 100 char excerpt.

you could also just do:

$content = $post->post_content;

if (strlen( $content > 100 ) {
    the_excerpt();
} else {
   the_content();