Generally, your choice will be made among 2-1/2", 3-1/2" and 4-1/2" louver widths. Perhaps the biggest surprise in store for you is that what you see when you look through your louvers depends on the width of the louvers you choose.
Looking through 2-1/2" wide louvers, your eye will be drawn to the louvers themselves because of the small space between them. Although it tends to obstruct the view, this narrow width is most often chosen when the majority of your windows are small, 2 feet high or less. A 2-1/2" wide louver allows roughly twice as many louvers in the panel as the widest width, and presents a more shuttered look. However, less light enters when your louvers are fully opened.
Looking through 3-1/2" louvers provides a dramatic difference in what you see. Your eye is drawn through the space between the louvers to the view outside. When open, this width allows more light to enter than the narrow width, and is ideal if the majority of your windows are 30" high or more.
Looking through 4-1/2" louvers, your eye is once again drawn through the louvers to the view beyond. When open, this width allows you to let in the most light, and provides the fewest number of louvers for any given window or door height. It is particularly popular for exceptionally tall windows and rooms with oversized ceiling heights, although it can be used anywhere."