How to set character limit on the_content() and the_excerpt() in wordpress

You could use a Wordpress filter callback function. In your theme's directory, locate or create a file called functions.php and add the following in:

<?php   
  add_filter("the_content", "plugin_myContentFilter");

  function plugin_myContentFilter($content)
  {
    // Take the existing content and return a subset of it
    return substr($content, 0, 300);
  }
?>

The plugin_myContentFilter() is a function you provide that will be called each time you request the content of a post type like posts/pages via the_content(). It provides you with the content as an input, and will use whatever you return from the function for subsequent output or other filter functions.

You can also use add_filter() for other functions like the_excerpt() to provide a callback function whenever the excerpt is requested.

See the Wordpress filter reference docs for more details.