Find empty pages or pages which have less than x characters

Here’s one way to do it:

Extending the post list table

Let’s assume we want to target the post post type.

The user interface we are after is the following:

where we can simply order by the post content length and then use the bulk feature to delete the posts that are empty.

We can achieve that in four steps:

Step #1

Add the Length column, next to the Title column:

add_filter('manage_post_posts_columns', function ( $columns ) 
{
    $_columns = [];

    foreach( (array) $columns as $key => $label )
    {
        $_columns[$key] = $label; 
        if( 'title' === $key )
            $_columns['wpse_post_content_length'] = __( 'Length' );     
    }
    return $_columns;
} );

Step #2

Fill that column with the post content length values:

add_action( 'manage_post_posts_custom_column', function ( $column_name, $post_id ) 
{
    if ( $column_name == 'wpse_post_content_length')
        echo mb_strlen( get_post( $post_id )->post_content );

}, 10, 2 );

Step #3

Make our Length column orderable:

add_filter( 'manage_edit-post_sortable_columns', function ( $columns ) 
{
  $columns['wpse_post_content_length'] = 'wpse_post_content_length';
  return $columns;
} );

Step #4

Finally we implement the ordering through the posts_orderby filter:

add_filter( 'posts_orderby', function( $orderby, \WP_Query $q )
{
    $_orderby = $q->get( 'orderby' );
    $_order   = $q->get( 'order' );

    if( 
           is_admin() 
        && $q->is_main_query() 
        && 'wpse_post_content_length' === $_orderby 
        && in_array( strtolower( $_order ), [ 'asc', 'desc' ] )
    ) {
        global $wpdb;
        $orderby = " LENGTH( {$wpdb->posts}.post_content ) " . $_order . " ";
    }
    return $orderby;
}, 10, 2 );

Notes

If you want to target other post types, than we just have to modify the

manage_post_posts_columns         -> manage_{POST_TYPE}_posts_columns
manage_post_posts_custom_column   -> manage_{POST_TYPE}_posts_custom_column
manage_edit-post_sortable_columns -> manage_edit-{POST_TYPE}_sortable_columns

where POST_TYPE is the wanted post type.