The longest historical tradition of contact balls comes from China, where they are commonly referred to as jian shen qiu (translating roughly to exercise/ fitness balls). They can be found in innumerable styles, but always with a connection back to the promotion of health and manual dexterity. Heavier solid metal versions are also commonly found in sporting goods stores, often under the name gong fu qiu (kung fu balls), as they have long been used to develop hand, forearm, and finger strength among practitioners. These contact balls extend this tradition. They are heavy and solid and are intended for relatively simple manipulation in the hands and fingers, in the Chinese style, and are NOT meant for free-flowing contact "juggling".
The 30mm size is quite tiny in the hand, but still very playable.
The 35mm and 40mm sizes are quite small in the hand and are probably best for beginners and those with small hands.
The 50mm size is a comfortable size for most adults and is quite versatile in that the balls are heavy enough to provide some resistance, while still being small enough to control with relative ease.
The monstrous 70mm balls are very large and very heavy, and not recommended for beginners.
Specs:
Stainless Steel
30mm Weight: 112/3.9oz g per ball
35mm Weight: 178g/6.3oz per ball
40mm Weight: 250g/8.8oz per ball
50mm Weight: 500g/17oz per ball
70mm Weight: 1400g/49oz per ball