how to get a window bigger than screen

I have a project where i need a bigger than screen window. I then use an other program to get the image from that window and remap it on different screens and projectors. i have already set the parameters i want (including resolution) in the build settings.

I have made a build of my "game", and it works as expected on ma personal computer: the window opens bigger than the screen. but when i run the same file on the installation's computer (connected to the screens and all), the window is resized to fit the main screen.

Why is that, how can the same build be different on different computers and how to get around this problem?

thank you for any help.