You will need two buttons, approximately 5/8” (1.4 cm). Rowan used brass buttons.
FYI - the second button is used to secure the front under-wrap section to the body.

Finished measurements:
Chest circumference: 20.75(21.75, 22.25, 23)“ 53(55, 56.5, 58.5) cm
Finished length: 9.25(10, 10.75, 11.75)” 23.5(25.5, 27.5, 30) cm
Sleeve to armhole: 2.75(3.25, 3.5, 4)“ 7(8, 9, 10) cm

Yarn used:
Rowan Summerlite DK
Shade 469 Favourite Denim - 2(3, 3, 3) balls