You're right that by default Nisus Writer tries to use your system paper size. But it's more precise to say that Nisus Writer uses the paper size as set in your Nisus New File template. The default template is indeed configured to use the system paper size, but if the template was ever edited or saved then it may specify an explicit paper size that isn't your system paper size.
Either way, the solution is for you to edit your new file template. Here's what you should do:
1. Open your Nisus Writer preferences.
2. Switch to the New File pane.
3. Click the Advanced tab.
4. Next to the label "Template File" you'll see "Nisus New File" and underneath its path on disk. Click the "Open For Editing" button.
5. The template file will open as a normal document window.
6. Adjust the template's paper size (and any other settings you like).
7. Save the template and close its window.
From now on all new documents you create should use the settings you established.
I hope that helps. If you want to see a screenshot of the preferences area described above see our FAQ on how to customize the default document window size and position
. Please let us know if you have any questions.