You will need 7 x .5” (15 mm). Rowan used shell buttons from Textile Garden in the UK.

Finished measurments:
Chest circumference: 32( 35.75, 39.25, 43, 47.75, 51.5, 55, 58.75, 64)“ 81( 90.5, 100, 109.5, 121, 130.5, 140, 149.5, 162.5) cm
Finished length: 19( 19.75, 20.5, 21.25, 22, 22.75, 23.25, 24, 24.5)” 48( 50, 52, 54, 56, 58, 59, 61, 62) cm
Sleeve seam: 17.25( 17.75, 17.75, 18, 18, 18, 18, 18, 18)“ 44( 45, 45, 46, 46, 46, 46, 46, 46) cm

Yarn used:
Rowan Summerlite 4 Ply
A. 436 Ecru - 3(4, 4, 4 ,5, 5, 5, 6, 6, 6) balls
B. 429 Navy - 2(2, 2, 3, 3, 3, 3, 3, 4) balls
C. 440 Langoustino - 1(2, 2, 2, 2, 2, 2, 2, 2) balls