packing nut size? (approx 2")

can anyone tell me, what is the size (AF mm) of the spanner I need to remove the co26 (1970s) propshaft.(vire 12 engine)
It has a large brass nut approx 50mm tight on the stern tube, inboard i cant budge with a pipe wrench
its very tight in there where the hull 'v' sand i need to know the spanner size

anyone help

Nick