So may be my question should be how to change the screen size showed in editor?
To change the screen size, go to the Game window (accessed by the menu “Window->General->Game”). At the top of the window will be a screen size menu (outlined in green, below) to change the screen size. When pressed, it will display the list of screen sizes that will be emulated. I say “emulated” instead of “displayed” because it is possible to have a screen size larger than the screen on which you are working. It is also possible to zoom into a screen. The “Scale” slider (to the right of the screen size menu) will allow you to scale up/down.
As a note, the list of screens and resolutions shown are related to the current build target (accessed by “File->Build Settings…”). For example, common iOS screen sizes are displayed in the list when the build target is iOS.