To set the window size in an Unreal Packaged game have these nodes in your Level Blueprint that are called on Event Begin Play:
Then package the game. In the Packaged Game folder Look for YourGameNameFolder/Config/DefaultGameUserSettings.ini
open the DefaultGameUserSettings.ini and edit it like so…
….Adding a custom ResolutionSizeX makes the BP above load and implement this setting. The FullscreenMode=0 . sets it to a borderless window in the top left of the monitor. IDK how to change the window position yet.