You will need 6 buttons. Rowan used # BN1150 from Bedecked in the UK.

Finished measurements:
Back width: 18.5(20.5, 22.5, 24.25, 26.5)“ 47(, 52, 57, 61.5, 67) cm
Finished length: 16.5(17.25, 18, 19, 19.5)” 42(44, 46, 48, 50) cm
Sleeve to underarm: 17.75(17.75, 18, 18, 18)“ 45(45, 46, 46, 46) cm

Yarn used:
Rowan Fine Lace
Cobweb 922 - 3(3, 3, 4, 4) balls