It could be a bug actually, but if you run your game in Window mode, the ScreenWidth() and ScreenHeight() expressions returns the actual desktop resolution in native game (in full screen it returns the game screen size).
So what you can do is, run your game in window mode at the beginning and get the desktop resolution using the expressions and store it inside a variable and then, change the screen size using the value of the variables and switch to full screen.
This way, the screen size is going to be the same as the desktop resolution but the actual screen resolution is remain the original and the screen is going to be scaled only. When you change the screen size, the option on the bottom “Use this size as default size for camera” should change the resolution (camera size) I believe but doesn’t work for me so the actual resolution is not changed automatically. but screen size
To change the resolution along with the size of the screen, you also need to change the camera size manually to be the same as the screen size using the same variables. But, in case you change the camera size more area of your scene going to be visible, so you need to keep that in mind when you design your scenes and GUI.
Finally, because you change the size of the camera, it position is going to be changed as well.
To solve this, get the X and Y position of the camera using the CameraX("",0) and CameraY("",0) expressions and store it inside variables before you change camera size. And apply the position after you have changed the size of the camera.
So the solution is looks like this in order:
First, get desktop resolution:
At the beginning of the scene:
Activate full screen : no
DesktopWidth = ScreenWidth()
DesktopHeight = ScreenHeight()
After, change screen size to be the same as the desktop resolution and then switch to full screen:
Make sure it is executed only 1 time and not every frame.
And make sure you change the screen size first and then switch to full screen.
Personally I hate it when games starts in small screen, move my gadgets and icons on the desktop and every time when I quit the game, I need put my icons and gadgets back where they were. To avoid that, change the screen size first.
Trigger Once:
Change screen size : DekstopWidth, DesktopHeight
Activate full screen : yes
After, change screen resolution to be the same as the desktop resolution:
Make sure it is executed only 1 time and not every frame.
Trigger Once:
CameraX = CameraX("",0)
CameraY = CameraY("",0)
Change Camera size : DesktopWidth, DesktopHeight
Camera X position : CameraX
Camera Y position : CameraY