Two questions: How can you set the default size of the game screen? and How can you set which game scene should be set first?
1: I’m assuming that you are talking about the screen size of .exe files. That can be found in the render panel, in the format tab where it says “Game Framing settings”. Just make sure that the camera is set to the same screen size or else it will look stretched.
2: The game will start on whatever scene is active when you save the file.