A size S slipover has a bust circumference of 108 cm [42½ inches] which in the given example would give you 18 cm [7 inches] of positive ease

The measurements for the finished blouse are listed on the front page of the pattern

slip 1 stitch

If you knit tightly

4 mm [US 6] double pointed needles

Knapper af galalit Ø18 mm - Sort - 8 stk. Broken rib A size S slipover hasButtons in galalith in packs of 8 pcs suitable for Jenny Jacket and Classic Smocking Cardigan Measurements: Diametre 18 mm Material: 100% galalith. Galalith is a natural and biodegradable material made from the milk protein casein. The buttons can be washed in the washing machine. Produced in the Netherlands. Galalith is a natural material which can shrink during the production of the buttons. The measurements are therefore approximate, and the