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:

1. Click the File tab
   
2. Click Export
   
3. Click Change File Type
   
4. Under Image File Types, choose JPEG File Interchange Format (*.jpg)
   
5. Click Save As
   
6. 
   a. Give the image a recognizable file name
   b. Click the Resolution Change button
   c. Choose High quality printing, and click the OK button
   
   c. Save the image to a folder where you can find it again easily
   

Now you're ready to go to Block Posters and upload your image:

1. Upload an image
2. Customiize your poster
   
3. Download the PDF of your poster
4. Print each 8" x 12" section
5. Put together on poster board or foam core

Hat tip: Print Posters With Almost Any Printer by Richard Byrne at his Free Technology for Teachers blog