The pattern calls for a C hook, but claims it is equivalent to a 2.50 mm size.

To fit chest sizes: 20, 22, 24, 26. ( 51, 56, 61, 66 cm )
Actual width: 22, 24, 26, 28 in ( 56, 61, 66, 71 cm )

Length: 11, 12.5, 14.5, 16.5 in ( 28, 32, 37, 42 cm

Additional materials: 7 1/4 in ( 5mm ) buttons