Impact test is for measurement of toughness of a material and not related to component and thus you may like to prepare the specimen of 10 mm X 10 mm X 55 mm and carry out the test. The test results can be directly correlated with your grinding ball . I suggest you follow the standard ASTM E 23 Standard for testing. Hope this answers meets your expectations.
Thanks you Sir, but I think that this method is like charpy testing. Really I need a impact testing over the product (grinding ball), not an standard method.