You can change the size of your canvas by clicking the Resize button on the top right corner of your Workspace.
This will open a dialog where you can type the new size you want. You can define the size in the following units:
Keep in mind
The units of the Workspace are always millimeters. So if you change the size to a specific measure in inches, on the Workspace it will be shown on its corresponding mm.
For example, resizing the canvas to 20x20", will result in a canvas of 508x508 mm.