Info from Kevin at KAB about the diameter of mats for the 1200.
" I am seeing lots of discussion on platter mats and I wanted to clarify some things. First some observations on the '1200 platter and the raised outer edge. The outer edge is raised only slightly, about 0.008 inch. The purpose of this is to ensure that the rubber mat puts some pressure on the outside edge. If you take the platter off, hold it in by the magnet below and strike it, it will ring. The only way to stop the ringing effectively is to grab the outside edge, touching anywhere inside that perimiter, is not effective. I have never suggested that mats should fit inside this edge, on the contrary, mats should fit over and onto this edge. So far the best system I have seen is the SDS Isoplat Mat 295mm and the thin tecnnics stock mat. these 2 give good overall mat height and stop all ringing. Do not use mats that are under cut, and always remember that you need a fair amount of weight as well to control the resonance in this platter."
" I am seeing lots of discussion on platter mats and I wanted to clarify some things. First some observations on the '1200 platter and the raised outer edge. The outer edge is raised only slightly, about 0.008 inch. The purpose of this is to ensure that the rubber mat puts some pressure on the outside edge. If you take the platter off, hold it in by the magnet below and strike it, it will ring. The only way to stop the ringing effectively is to grab the outside edge, touching anywhere inside that perimiter, is not effective. I have never suggested that mats should fit inside this edge, on the contrary, mats should fit over and onto this edge. So far the best system I have seen is the SDS Isoplat Mat 295mm and the thin tecnnics stock mat. these 2 give good overall mat height and stop all ringing. Do not use mats that are under cut, and always remember that you need a fair amount of weight as well to control the resonance in this platter."