The size of the layout is the white canvas, you change its size at the layout properties menu. The size of the screen however (which is the dotted retangle) you can change at the project properties. Just click at your project folder at the menu on the right and you'll be able to see it.
About the goal thing. It's quite simple, all you need is an object to be your goal, it can be a castle, a door, a sign, a flag or even nothing. Just place a invisible object at the end of the layout and it shall do the trick.
Set a condition to trigger when the player collides with this object, then you can either call the function "go to next layout" or you can use "go to layout "name" ".
Hope it helps