Well, this is going to be a very "rough" estimation. I'm going to measure the cirumference using the bottom reed plate as part of the curve. The reed plate will be in the same position as it would be if it were holding the reed in place on the mouthpiece.
Cicumference = TT x Diameter
The Diameter (from the top of the lig to the closed bite plate) is approx 3/4". Using 3.14 for TT, the circumference turns out to be approx 2 1/3".
And if you need the metric measurements;
Diameter = approx 2cm
Circumference = approx 6 1/4cm