The layout size is infinite, you can scroll forever, and it doesn't affect performance. You should only really be concerned about the number of objects, especially the objects that use large images and take up a bunch of memory. Objects that are off screen are not rendered and hardly affect performance. Just take into account the total memory usage for that layout and try and keep it under 100mb. I would say best practice though is to do what you're trying to avoid and mimic the scrolling, creating a large level isn't too useful although you can make use of the tilemap object that helps to keep memory usage down when drawing a level.