You can use standard size of your design area (the default mode) or maximize it to full workspace. The custom size can be set as well.
To set standard predefined size - press from the toolbar.
To set maximum size - press from the toolbar.
To set custom size:
You can build your design in landscape or portrait modes (orientate 'building ground' vertically or horizontally).
To set portrait mode - press from the toolbar.
To set landscape mode - press from the toolbar.