render without displaying window and resize image

Hi all,
i am an newbe to OpenGl. I would like to use OpenGL to render a scene. The scene hasn’t to be shown in a window, so no visualization of the scene is needed.
The rendering should be triggered from the program and not run in an whil(true) loop.

I would like to render a szene with OpenGL,
load the picture from the buffer with glReadPixels,
resize the display to a new size. render a new scene,
load the new picture with new size from the buffer with glReadPixels,
etc, etc

Here my questions:
How can I use OpenGL without a open window?
How can I set/change the displaysize in the program for the next szene?

Many thanks for reading so far, hope someone has an idea.

Best regards,

Are you trying to say that you want your scene to be fullscreen or that you want to use OpenGL to render a graphics scene but not create a window to display it?