When you want to make a large poster on your regular printer, try the free online service Block Posters. You can upload a graphic, then divide it into 8" x 12" chunks for printing. Print out each section and put them together on a poster board to make your own poster.
Note: if you've created your poster in Publisher, you'll want to follow these steps to save your document as an image first. Here's how: (Joy's blog)