How to change canvas size in unity3d
So may be my question should be how to change the screen size showed
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.
Use the Canvas Scaler Component