Can I make the captcha more complicated looking or longer?

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 );

Same here –
It generates the image with maximum 4.5 symbols.
Can we do something about it?

Also interested in a way to do this.

count me in as well

Set the size to Large using this tag in your Contact Form 7 form:
[captchac your-captcha size:l]

In Really Simple CAPTCHA change the char_length to 8 in really-simple-captcha.php:

/* Length of a word in an image */
$this->char_length = 8;

There seems to be a bug in the img_size parameter in really-simple-captcha.php.
The Width settings is ignored so I hard-coded the setting.

Change:
if ( $im = imagecreatetruecolor( $this->img_size[0], $this->img_size[1] ) ) {
To:
if ( $im = imagecreatetruecolor( 184, 28 ) ) {

The Width of the CAPTCHA image is also hard-coded in Contact Form 7.
So you must also edit the file: modules\captcha.php

Change:
case 'l':
   $op['img_size'] = array( 84, 28 );
To:
   $op['img_size'] = array( 184, 28 );