A short function to limit WordPress blog title to 40 symbols.

limit_blogname.php

<?php
  add_filter( 'pre_update_option_blogname', 'arunas_limit_blogname', 10, 2 );
  function arunas_limit_blogname( $value, $old_value ) {
    // check for length (pr do any other kind of validation)
    if ( 40 < strlen($value) ) {
      // throw error via Settings API
      add_settings_error( 
        'blogname',  // option name 
        'blogname',  // error id
        __( 'Blog title is too long', 'plugin-slug' ),
        'error' 	 // error type
      );
      // short-circuit `update_option` to prevent new title from being saved
      $value = $old_value;
    }
    return $value;
  }
?>