I don't recommend matching your document pages to your screen size for two reasons: 1) the app's pagination functions only work with standard Letter or A4 page sizes, and 2) if you get a new device, you'll have to redo all your documents.
Instead, while viewing a document in BandHelper, you can click the document tools button (wrench icon) and select Fit to Height. Then the document viewer will resize to fit one complete page at a time, and the next/previous page functions will scroll one complete page at a time.