The size of the artwork will be A3-sized landscape (420mm x 297mm) on 300dpi
you can start sketching if you want to. Don't worry about the vacancy. We'll fill that up later.
As for the deadline, we don't have any official date yet, so have fun with your sketching )
We will be following this templates right here that was suggested, so there should be an empty space for the dates to go in your drawing. We'll tell you later the details about it.
In the mean time you may go experimenting about.