Hi Rob - they are 200mm squares, in 3mm card. With about a 2mm gap in-between tiles. It was a bit of a guess but based on Gordon's Lancaster diorama on pages 17-18 of collector 22. Using card does mean you get a little warping when applying wet polyfilla, but it's a lot cheaper than using 2 or 3mm plastic card and looks OK.... Cheers. jameson