MickeyRoush on "[Plugin: Contact Form 7] Word Count Restriction"

You can limit it to characters with something like this below. Just add it to your theme's functions.php and change the 400 to whatever you need.

// Limit Contact Form 7's textarea/message field to 400 characters
add_filter( 'wpcf7_validate_textarea', 'character_length_validation_filter', 11, 2 );
add_filter( 'wpcf7_validate_textarea*', 'character_length_validation_filter', 11, 2 );

function character_length_validation_filter( $result, $tag ) {
	$name = $tag['name'];

	if ( ! $result['valid'] )
		return $result;

	if ( 400 < strlen( $_POST[$name] ) ) {
		$result['valid'] = false;
		$result['reason'][$name] = "You have exceeded the 400 character limit.";
	}

	return $result;
}