Thanks for the answers. I don't think there is a definitive answer out there and even if there was it would have to be updated over time.
I did find by searching SCF and following a link above some answers:
looks like a Mongolian stamp is the largest (I found): 5000T Peace MANDALA 185x140mm
The China stamp Don posted is one of series of Chinese Imperial multi-part stamps issued from 1905-12. They seem to all be 5 part stamps with only one part going on the actual mail piece and the other parts used as receipts to various parties. The one Don posted is missing the first two parts. Complete unused examples are quite valuable.
There is a $10 Canadian Whale stamp that is 130x50mm
There is a Yemeni Cats stamp that is 95x108mm but this may not be an "officially issued" stamp
There is a 2006 Tuvalu Rembrandt stamp 75x106mm
There is a Vietnamese stamp 90x120mm
One last one I found after initially posting this is a Dutch (Netherlands) "stamp." Not a single stamp but a single image spread over a whole sheet of stamps.
Probably others I have missed.