When working on a device the forms screen size is same as the physical screen size and
you can't control the forms size.
On the desktop you can change the "screen" size.
Changing the screen size will affect all forms.
Changing is done through the Visual Designer - Tools - Screen Size.
For example, if you want to write a landscape application change the screen size to:
Height - 240
Width - 320
These settings won't change the screen orientation on the device automatically.
Using Form.Width / Form.Height you can get the size of the user available area (without
the menu and tittle bar).