Limit the content entered by editor or admin in the WP admin editor window

Checking to length and limiting it is no problem, but as you set the category when you save the post setting different limits per category would be a bit trickier. Below are the steps you would need to take:

• checking the $post var
• make it mandatory that a user selects a category
• seeing what cat the user chose
• setting custom category meta data containing the max length for that category
• and then only submitting if it is less and if it isn’t throw an error

But as I said just setting a general limit is fairly simple. Here is a simple snippet of PHP and javascript that retrieves the current editor on the page, counts the number of characters in the editor and displays a warning if the character count exceeds a certain limit. Using a prominent warning instead of truncating content automatically offers a much better usability. You could always make this into a plugin to make things simpler if not make sure you are using a child theme.

I haven’t tested this, but it should work fine.

add_action( 'admin_print_footer_scripts', 'check_textarea_length' );

function check_textarea_length() {
    ?>
    <script type="text/javascript">
        jQuery( document ).ready( function($) {
            var editor_char_limit = 50;

            $('#post-status-info').append('<span class="word-count-message">Reduce word count!</span>');

            tinyMCE.activeEditor.onKeyUp.add( function() {
                // Strip HTML tags, WordPress shortcodes and white space
                editor_content = this.getContent().replace(/(<[a-zA-Z\/][^<>]*>|\[([^\]]+)\])|(\s+)/ig,''); 

                if ( editor_content.length > editor_char_limit ) {
                    $('#post-status-info').addClass('toomanychars');
                } else {
                    $('#post-status-info').removeClass('toomanychars');
                }
            });
        });
    </script>

    <style type="text/css">
        .word-count-message { font-size:1.1em; display:none; float:right; color:#fff; font-weight:bold; margin-top:2px; }
        .toomanychars { background:red; }
        .toomanychars .word-count-message { display:block; }
    </style>
    <?php
}

If using this in production, remember to move both Javascript and CSS into files and use appropriate admin hooks for adding them only on pages that display the editor you need.