I know 2 methods to identify own paper size.
1. For windows XP:
Start -Settings -Printer&Fax -File - Server Properties -Create a new form
After adding your new form, you'll see it in pdf24 printer page settings
OR
2. For example you have word-file. Choose File-print, choose Pdf24-printer and click Properties - Paper/Quality - additionally - Paper Size - find smth like "Induvidual page size PostScript - add your parameters.