Hi Giovanni,
I have created a template for you. All that you need to do to create a template (or modify an existing one for future use) is to follow these steps:
1) Modify the page size in File > Page and Print Setup to 13 in x 19 in
2) Double click on the data frame (Layers) and on the "Size and Position" tab change the Width and Height to 12.5 in x 18.5 in.
3) Save your map in the following location C:\Users\YOUR USERNAME\AppData\Roaming\ESRI\Desktop10.0\ArcMap\Templates (on Win 7) or C:\Documents and Settings\USERNAME\Application Data\ESRI\Desktop10.0\ArcMap\Templates (XP).
Now, you can select this template from the "My Templates" section of the New Document Wizard.
Please download this template and store it in the appropriate location from step 3.
Best,
Juliana
