The layout size and window size are two separate things. (You may know this already so sorry if I'm misinterpreting the question.)
The window size might have to change to fit different devices, but the layout size probably doesn't.
If I recall, The layout size is only really relevant for behaviors like "destroy if outside layout", and for convenience while placing objects to make up your game world. As long as the layout is big enough for you to work comfortably, then you should be okay.
For instance, I tend to set the layout size to be pretty large, and most of the time I don't use all of it. The only thing an end user will see is the part of the layout that shows up in the game's window.
One of the other features of layout size that might be important, is that if you scroll your view around in your game, the game window will stop at the edge of the layout by default. You can free up the window to scroll off the edge of the layout by clicking the layout in the project panel, and in the layout's properties, you can enable "Unbounded scrolling".