Found this information at
How to Determine TriClamp Size
TriClamp Size is determined by the outside diameter of the pipe, NOT the outside diameter of the ferrule. The ferrule ( the end piece that makes half of the connection to another TC fitting) is about 1/2" larger than the TC size. In other words, a 1.5" TC fitting will have a ferrule whose outside diameter is about 2". Confusion arises with 1" TC fittings because they use the same ferrule as 1.5" TC fittings. [The two ferrules at right in photo are 1.5" and 1" Triclamp.]
They also have a chart of sizes on that page. looks like 2.5" would be your best bet the lip on the Guinness keg is really wide compared to standard sanke.