Since the Z-axis weighs 6g=58.8N and the maximum force is in the opposite direction, the magnitude of the maximum force (from the information given) is 58.8N, as |58.8-100|<58.8N. That's a very small force, so I doubt you need to worry about the diameter of the leadscrew, unless there are other axial forces on it, which I guess there would be since this is probably for your coffee grinder? The easiest way to be sure is to check the manufacturers load ratings for the screw in question.

For small screws with low forces it's common to use the smaller angular contact bearings, which are often called magneto bearings. For example:
Angular Contact Ball Bearings - Arc Euro Trade

You would almost certainly need a pair of them.