Depending on your needs your crystal sphere may vary:
Healing on-the-go: You want a small, portable sphere. You should consider a sphere that fits in your pocket for portability. You may even want something smaller like a crystal sphere bracelet You never know when you'll need an emergency crystal ball reading!
Massage crystal balls: You want something that fits comfortably in your hands. It should be palm-sized so you don't lose control when you are rolling it over the body.
Meditation crystal balls: You want two spheres, one for each palm of the hand. Alternatively, one large sphere that will fit between both hands.
Feng Shui balance: You want a crystal ball that is relative to your space. For example, if you are filling the energy of a studio, you would choose a different size crystal ball to fit the home than if you were filling the energy of a mansion.