I am trying to create a lunch screen image for my iOS app. But I am not sure whats the right size for it.
According to apple website for iPhone 6 the size should be 750
But my question is, how to create a launch screen image that will fit for all screen sizes such as iPhone 5, 5s, 6, 6 plus, iPads etc?
That will be very hard to do. The devices have different aspect ratios and the difference in size between say iPhone 5 and iPad Air is pretty big to use one image for all. If you want to use the classical launch screen with a screenshot, resizing one image won't look the same as the user interface on different devices. If you want a launch bitmap image you will need to supply all the necessary sizes.
Whats the correct way of doing this?
You can create either launch PNG image or launch screen as a storyboard.
For launch image, your project has an assets catalog. Either use existing or add new iOS launch image. The editor will show you all required sizes.
For a storyboard, your application probably already has LaunchScreen.storyboard. You typically won't create specific size or sizes here, but use auto layout that will resize the screen appropriately.