Changing the page layout and canvas size in LibreOffice Draw

You can use the menu Format/Page dialog to configure the size and orientation of the canvas.

MenuBar->Format->Page/Slide properties...

I suggest changing to screen orientation and playing with the size to get plenty of room for creative thinking

In LibreOffice 5, it is under the Format → Page/Slide Properties... dropdown:

This will open the Page Setup Dialog. Under Paper Format, you can choose Format, Width, Height and Orientation (Portrait/Landscape).