My double-decker coop is similar in size. Originally, I had two vents on opposite sides, high up, for cross ventilation. Each opening was about 8 square inches. It was not enough. I cut a window about 8 inches wide and 18 inches tall. It made a huge improvement, not just in ventilation. The birds sleep in front of the new opening, even when it gets chilly. They use the space during the day, because they can see better. With early morning light, they lay more often despite the shorter winter days.
I'd say start with about a 4 x12 inch vent on each side. If you don't have a window, go little bigger. The birds should have enough light to find their way in the daytime and enough air that you never have a burning sensation in your eyes when you stick your head in!