The size of the picture window you can install is limited by the size of the opening. It can be smaller than the opening, with casement windows on either side to provide ventilation, but it cannot be larger without extensive reengineering and remodeling. Some manufacturers have a maximum size they offer as well, typically 72 to 96 inches in height and width. At Zen Windows Milwaukee, we can size your picture window perfectly for your opening and provide a professional window installation.