The Width and Height of a web page can be altered from the Page Size tab in Options (Select Utilities followed by Options). You can select a default page size from the drop list or select Custom and specify your own width and height.
The page height can also be adjusted by clicking on the selector tool and placing your mouse pointer on the bottom edge of the page. The Mouse Pointer will change from an arrow. Left click and drag the page up or down to alter the height of the page. Note you may have to re-position some of the items on the page.