Bacci make their own friction plates and they do them in various sizes for different applications. I got 4 last week but they were too big, probably for some older BMW diff. I sent them back along with a used Alfa disk for reference and called them on the phone. Giulio said the big ones are what they use for Alfas but I very much doubt this, they probably just packed the wrong stuff. They also have something very clever which I have only seen in ZF BMW LSD's: flat plates shaped as conical springs for applying constant preload, used one per side, on the end of each clutch pack. With these, adjustment doesn't have to be so critical any more. I've also ordered 4 of them! They're the same price as the friction ones, less than 20€ each.
Back to your question, they said they can supply the same size as ZF so all is well, as this is what I want them for anyway.