One of the main objectives of the Layout Designer is to make it easier to design a layout that will adjust itself to fit different screen sizes. If your layout is not fitting on the screen then you are definitely doing something wrong. There is a guide to using the designer
here and there is a YouTube video
here. If these do not help you then try posting a simple layout on the forum and someone will probably show you where you are making mistakes.
Fitting a layout to different screen sizes is to a large extent achieved by using "anchors" to set the positions of the views relative to the screen edges. If you have not used anchors then that is probably the reason that you are having problems.
Here is a very simple example. This layout has a label at the top of the screen, two buttons anchored to the bottom corners of the screen, and a panel that fills the area between them. They all use anchors to set their positions.