Word for WPF gives you the flexibility to create a word document with different paper sizes. You can use PaperKind enum to specify any of the available standard paper sizes.
Note that a class named WordUtils is used in the code given below. It is available in the product sample located at the following location on your system:
Documents\ComponentOne Samples\WPF\WordCreator
You can use these classes in your application from the mentioned location.
The implementation of PaperKind enum is given in the following code: