I'd think that the beautiful Connoisseur £1,000,000 plaque is the largest plaque known although it was just for internal use and so never used on the tables. Just as well really as no table had a large enough amount in the float to cover it being bought in with.
As the Clermont gold plaque was 7" by 4" (or 4" by 7" as they described it) I wonder if the Playboy one was comparable (or not) in size.
I'm not sure how big the Clermont £10,000 tortoiseshell one was but quite close I think.

The Le Cercle £5k and £10k ones are also 6" by 4" so maybe that was a standard size for some higher denominations. I don't know how many were made, maybe just the 10 that you've mentioned for other similar ones.

Extra Playboy information from another book by Victor Lownes. No idea why it is sideways but I tried inserting another one to counteract it and it didn't work so ........ I'm sure there is somebody that can fix it.

And from a member of staff that may have mixed up the plastic ones due to her saying that there were multiple gold ones so she could have actually meant the plastic ones. Or maybe she just added an s by mistake (or was misquoted).

Here's hoping the photo can be found.
I also look forward to seeing the White Elephant and A.M white plaques too.