In any cmm software when you define your calibration sphere, a stem diameter (the "neck" of your artifact) is required to allow software calculate clearance moves and touch points around the master sphere to avoid collision. Typically you can just measure it with vernier caliper and enter the number in. It seems this parameter is not that important as others, but I always tell my trainees to enter a slightly larger value than the actual size because this will make software think your master sphere has a larger "neck" thus it will create more clearance between your probing points and the stem. In many cases this will help a lot when you have a long probe configuration and need a sensor angle such as A105, B-52.5. Larger stem size won't hurt anything else.

In Open Dmis:



In CMM Manager: