Is there a way to reduce the size of the captcha box?

You can make changes to the settings in this file: 

/components/com_breezingforms/images/captcha/securimage_show.php 

If you want to change some of the styling for the Captcha box, you could also check the bfCaptcha settings in this file: 

/components/com_breezingforms/themes/quickmode/system.css

//Change some settings
$img->image_width = 230;
$img->image_height = 80;
$img->perturbation = 0.9;
 
$img->image_bg_color = new Securimage_Color("#6495ED");
$img->text_color = new Securimage_Color("#B0E0E6");
$img->line_color = new Securimage_Color("#B0E0E6");
$img->noise_color = new Securimage_Color("#B0E0E6");