Gentlemen, for the Big Guff I am using a 1200mm x 600mm piece of plasterboard, plaque de platre in French, which is indeed made out of gypsum. That's about 4 ft x 2ft in Imperial measurements. Because of the length of the Big Guff's fuselage I've had to add a bit extra as the pictures show. Glue will mark the surfaces but I find that even a used board is flat enough for my standards! When they become totally unserviceable they are cheap enough to replace.
For smaller models I have a piece of board, such as were used for school noticeboards, glued to a sheet of chipboard, as a building board. I suppose that all notices are posted electronically on computers, iphones and tablets at today's schools!