New document (artboard) can be created by choosing File → New from the main menu, or by pressing CTRL + N shortcut.
In the following dialog the dimensions of the artboard can be set.
Dimension can be quickly set after selecting one of the presets, or by manually entering values.
By clicking on the Refresh button at the bottom of the window, a code will be generated. The code can be then copied into Clipboard or saved as a file.
Read next: Shapes