But my question is how to create a launch screen image that will fit all screen sizes such as iPhone 5, 5, 6, 6, iPad, etc.
It will be very difficult to do. The devices have different proportions, and the size difference between the iPhone 5 and iPad is large enough to use a single image for all. If you want to use the classic launch screen with a screenshot, resizing a single image will not look the same as the user interface on different devices. If you want a bitmap image to launch the image , you will need to specify all the necessary sizes.
What is the right way to do this?
You can create either launch a PNG image or launch the screen as a storyboard.
Your project has an asset catalog to run the image. Either use an existing one or add a new iOS launcher image . The editor will show you all the required sizes.
For storyboards, your application probably already has LaunchScreen.storyboard. Usually you will not create specific sizes or dimensions, but use an automatic layout that will resize the screen accordingly.