You need to specify a screen size like, for example, 800, 500. (this is just an example)
You would do that under Properties (and the look down a few lines until you see) Window Size.
There, you specify what you want.
A better way is to start with a template which you can find in the File > New > pop up window
Select a Template or example.
Then you will see a dotted area which is your screen size.
What ever you put in there, will appear on the screen right where you want it to be.
Check the Layout Size also (select layout in the Projects window) and make sure that it is set to the same size.
Btw. GREAT graphic!
Good luck with your project,