You will need 3 buttons. Rowan used # G440428 from Groves & Banks in the UK

Finished measurements:
Back width: 17.5(20, 21.5, 24, 25.5)“ 45(50.5, 55, 60.5, 65) cm
Finished length: Approx 20(20.5, 22, 22.5, 23.5)” 50.5( 52, 55.5, 57, 59.5) cm
Sleeve to underarm: 17.5(17.5, 18, 18, 18)“ 45(45, 46,46, 46) cm

Yarn used:
Rowan Alpaca Soft Dk
Shade 210 Rainy Day - 9(10, 11, 12, 13) balls