You will need 3 buttons. Rowan used #BN1470 from Bedecked in the UK.

Finished measurements:
Back width: 20.5(22.5, 24.5, 26.5, 28.5)“ 52(57, 62, 67, 72) cm
Finished length: 13.5(14, 15, 115.5, 16.5)” 34(36, 38, 40, 42) cm

Yarn used:
Rowan Cashmere Tweed
A. Andorra Red 106 - 4 balls
B. Oats 001 - 4 balls