Right click anywhere on the drawing area and choose Page Setup from the context (popup) menu to show the Page Setup dialog. In the Page Setup dialog the size of the drawing area can be changed. The dialog can also be reached from the File menu (File->Page Setup).
This is also described in the documentation. Open Documentation Center (Help->Documentation Center), search for resize, click the first search hit. Scroll down on the page and right before the end of the page it is described under Resizing Graphical Layers.