My pot is about 15" across and the thermometer uses about half that diameter.
So there is a probe that goes into the center of the pot? Just trying to picture it. The chiller I bought on eBay is 9" in diameter, the height of the coils and spacing is adjustable since it's soft copper you can just carefully bend it.
Another option you may want to look into would be a plate chiller, that would chill the beer as you passed it through the device.