Balance is everything. These can be paired with 20x20 cushion covers or a pair of accent pillow covers to break the monotony and also make things visually appealing. You need not have many of them; a couple of good posts will work wonders in altering the entire atmosphere of your room.
Concerning material, it actually depends on your lifestyle. Cotton is easy to wear and breathable, linen has that slightly raw, high-end feel, and linen blends will be ideal when you need to wear something durable without losing the softness. Block print designs come in a huge variety of color choices, and can be ordered in any size you desire, making it easy to make them fit something: a bed, a couch, a window seat, or even a small corner of a room.
Imagine these covers as the icing on the cake, small yet effective.