I want to build Shoji screen door and need to set a frame from lumber pine to hold the door. Door width 5.5 meters(18 feet), height 2.5 meters (8.1 feet) Between the left and right lumbers(blue ones) is a wall(concrete), above the upper lumber(Yellow) is air (the height of the room is 3.75 meters), the upper only supported by the left and right lumbers(blue ones). I do have 6 meters long lumbers with 10x10cm or 15x15cm or 20x20cm or 25x25cm. It's inside the house(above the yellow air, above the air roof, no snow :)), the only purpose of this construction is to hold the Shoji screen sliding doors. The lumber is pieces of glued pine. So, what is the lumber thinkness should I use? 10, 15, 20, 25 cm? Thank u all, and sorry for lake of explanation.
Since it is not a load bearing opening, and all it needs to do is hold the doors up a much smaller header would be required. it is still a long gap to span and the doors will have some weight. the 15x15 would probably do fine, but I would recommend bumping it up to the 20x20, and going with the 25x25 you certainly wouldn't be unhappy with that.
To recap, I think the 15x15 might just pass muster, at least for a little while but since you have bigger options ready, I would go with one of them.