You can set the layout widths and heights for each layout.
Open the desired layout, and on your left you will have "Layout size" and enter the desired size.
You have 2 size indicators in construct 2.
1. is your project view window, the actual size it will try and show in the browser/device.
2. is the layout size.
The project view window is like a window on top of your layout, and will only show the bits the window size allows you to see.
So for instance, if you have a project window of 800 wide, and a a layout of 1600 wide, the project view window should only show half of your layouts total width.