It's a tough one. The ///M exhausts are 60mm I'd and 64mm od. There is just about 4mm between them so in theory you could only go as far as 65mm od and still leave 2mm 'rattle room'.
Mine are 89mm. Doubt you would want to go bigger than that as they pretty much fill up the cut outs. I think 83mm are what the eismann(sorry if spelt wrong) quads have and they look good.