You must be familiar with ASCII art where an image is converted into one that is created by using only text and characters from the keyboard. You can find lots of ASCII generators on Google. These generators usually produce ASCII images that are of the same size as that of the original image or sometimes slightly larger. These aren’t suitable if you intend to make posters out of them.
ASCII Poster is the one you should use if you want to make poster sized images. ASCII Poster is an online service that will blow up your image, divide them into A4 or letter paper sized pages and provide the output as a PDF document which you can print and layout the pages to produce that big poster.
Just upload your image, choose the paper size, see a preview and download it.
The preview of my poster. If printed, this 500×500 pixel image will become a 2 feet X 2 feet poster.
A small section of the poster