My personal preference is to not have real bright coop. One of the possible causes of aggression is too much light. I've seen that comment in articles about commercial hen houses and brooders. So I don't like to overdo it but how much of this is just rationalizing my personal preference? If I liked it bright I could probably come up with reasons to support that view.
You and the chickens need enough light to get around in there and I prefer to do that with natural light. My coop is a bit larger than yours, roughly 2.5 x 4 meters. I only have one standard sized window on the north side (away from the sun) but I also have several areas cut out and covered by hardware cloth that admit light.
Like Dobie, mine are hardly ever in the coop during the day. I don't know how much time yours will spend inside, that will depend on what your run looks like and your climate.