Basically, if you create a 20"x30" document in Posterino, you should make sure that your output is 20"x30" as well or of the same aspect ratio as your document.

To ensure that your document is not cut off:

1. Gather information about the output media size:
   • Visit the web-site of the print service you plan to use
   • Ask the personnel of your local print shop
   • Inspect the printer page setup
   • ...
2. Make sure your document size corresponds to the print media size gathered at step 1:
   • Open the Adjust tab (Command-Shift-I)

   • Inspect the settings on the Page Size panel:

     

     The page size settings.
3. If necessary, change the size of your document to make it corresponds to the output media size.