I'm no print tech, but the print techs who do hang out here all say to design at a percentage of full scale.
If you're doing large-format printing, you should probably absolutely talk to your print service provider first.
i'm not doing large format printing, the words are set to 18pt size i just need the document to be up to 400 inches in length. i have an hp designjet 111 roll printer, as such i will be doing my own printing. i just need to know how to make the pages print consecutively with no page break, such as in a paper banner.
I don't want to speak for Joel, but what I think he is suggesting is that you set up your document at half the size, and print it out at 200%. Your width would be 200" instead of 400, and your type will be 9pt instead of 18. When printed at 200%, it will be 18pt type in a 400" document. If you have any raster images, set the resolution to twice what you need it to be. Purely-vector images don't matter, as they will scale without resolution problems.