I too would like to make a longer and more complicated captcha. It appears some scripts are able to bypass the current length and spam through Contact Form 7. I did try change the character length from 4 to 10:
/* Length of a word in an image */
$this->char_length = 4;
The image seems to disappear after 4.5 characters and I could not get the length any longer, even editing this section:
/* Array of CAPTCHA image size. Width and height */
$this->img_size = array( 72, 24 );
