I had a similar issue making the world size too large.
If the greenfoot application itself is open but and you cant see anything and you're on windows, try hovering over the green foot icon in the taskbar. A small rectangle containing a miniature view of greenfoot should appear, right-click on it and press maximise.
Otherwise, if the application itself isn't opening, you can go into the file location where the greenfoot projected is located. Look for the "MyWorld.java" or whatever you named the world (WorldNameHere.java), right-click it and press open with notepad. You should be able to edit it there.
Yep, go to the folder of your scenario, go to the java file of your world and as explained by rocket open it with notepad, and when you see "super(3000, 700)", change it to like "super(1500, 700)" or less long