For calculating field outside a charged plane conductor, a gaussian surface of Cylinder is considered. I have been said that we consider cylinder because the circle in its upper end is in equal distance to the charges on the circle of its lower end. This way electric field intensity on the upper surface is equal at all points of that area.
I am particularly confused over why couldn't there be a cube instead of the cylinder. Is there a mistake or misunderstanding with how I am understanding choice of gaussian surface?