12 - Limits
Your webserver (probably (hopefully) Apache) may have some limits as to how much information can be transferred via CGI's POST method. These limits directly affect how much information you can put in any of wtfm's form fields. For instance, the default limit for Apache 2 via GET is 8190 bytes; so basically, if your POST limit was similar, you wouldn't want more than 8000 or so characters in any form field.
From what I understand, Apache limits POST requests to about 10 megabytes, so in practice, at least with Apache, there should be no problem. Other webservers, however, may behave quite differently, so be aware of the possibility.
Having said that, if you absolutely have to, you can use the system's include file capability to include anything, of any length. You'll just have to use an external text editor to prepare the file; you can still use any technique, variable or style thing you are able to use in the Page Content section of the system forms.