The planes are not made from cardboard, rather they are made of foamboard, light plastic foam sandwiched between thick paper. The foamboard that Flitetest sells is approx 4.8mm thick and is very lightweight making for super flying models. Foamboard available in the rest of the world tends to be 5mm thick and the paper is noticeably thicker making it a lot heavier overall. You could have the full plans printed on A1 paper at a commercial office supplies company but most print the tiled plans on A4/Letter sized paper and tape or glue them together to make the full sized plans.
The earlier plans are varying size. however the plans for the last 3yrs or so have been ANSI D. (if you print full sized.) Most of us here don't have access to large plotters. Good news! You can have Adobe acrobat print as any size you have, when you select the Poster option under the Page Sizing & Handling section in the print dialog box.
This is the sole reason I only print as full sized plans for my designs.