One of my major wishes is a 1:35th scale model of the German battleship the KMS Bismarck, which will measure over seven metres. I tried an experiment some time ago, and constructed one of the main 15" gun turrets, and can say that the finished product is a wonder. I will post some photos tomorrow morning.
My basic point is, that size does not matter. If the structure is reinforced well and the stiffeners placed where they should be; than there is no problem. It is very important - DO NOT USE PVA (WHITE GLUE) WITH CARD BOARD - the best adhesive available to be used is UHU adhesive - the best in town. White glue is water based and tends to distort the cardboard, while UHU, adheres well and creating s very sound joint when dry without any side effects. it can even be sanded down.
Anyway fasten your seatbelts, as more is to come these coming days.