To define a custom paper size:

1. Open the preferences window by choosing Preferences... from the Posterino menu (or press Command-",").
2. Select the Sizes toolbar button and the Paper Sizes tab:
   

   Templates details.
3. Press the button to add a new paper size.

4. Have a look at the existing paper sizes and define something similar for the paper size you need.

5. The next time you create a new document you can choose your paper size from the "Size" popup menu in the template chooser:

   

   Custom paper size in the template chooser.