The way it's set up in the sample project, the editor passes a command-line switch to the game called devmode. When this is encountered, the game automatically launched into a 1280x720 window (multiplied by the Windows DPI scaling value). The idea here is that if you are launching from the editor, you probably want to be able to switch back and forth between the editor quickly, either for rapid playtesting or for using the debugger.
This code is all found around line 65 of the code file "Scripts/Game/Game.lua".