How to choose the resolution of a video.

Hi !
I would like to do a video in 800 by 800 pixel but I don't understand how to choose those dimension. I know that in the rendering section you can change it before starting the render but I can't choose the part of the screen I want. Could someone help me please ?
Thank you.
I would like to do a video in 800 by 800 pixel but I don't understand how to choose those dimension. I know that in the rendering section you can change it before starting the render but I can't choose the part of the screen I want. Could someone help me please ?
