I need to change the font size of an image CAPTCHA phrase.
Login as Joomla administratrator.
Select Extensions and Module Manager
Select the Simple Email Form
Select Advanced Options
Locate Captcha Font Size
Set a value (48 px shown) and save.
Refresh the web page with the Simple Email Form to see the result:
This is what font size 24 looks like:
NOTE: this only applies to Image CAPTCHAs!!!
Please note that if you plan to use a CAPTCHA and you are using the Simple Email Form as a module, you will need to disable caching. Otherwise you will always see the same CAPTCHA image! If you are planning to use the Simple Email Form in an article, and also plan to use CAPTCHAs, you will need to disable caching on the article. This can be accomplished using the JotCache Extension.
http://extensions.joomla.org/extensions/core-enhancements/performance/cache/13155 for more info.