On (some) balls, the needle size is given as 3.5 to 4 mm (ball bought in July 2015, Germany). The Gründl website gives 3.0 to 4.0 mm.

Gauge: 22 sts x 29 rows = 4” square.

Machine wash 30° C.
Do not bleach.
Do not tumble dry.
No iron.
Dry cleanable (underlined P).