If you make your project with a Unity Personal subscription, a default splash screen displays while your game loads. It is oriented according to the Default Screen Orientation option in the Player settings. You cannot change this splash screen.
Unity Pro subscription users can change the splash screen. You can use any Texture in the project as a splash screen.
The size of the Texture depends on the target device:
The Texture you supply is scaled to fit if necessary.
You can set the splash screen textures using the iOS Player settings.