Thanks for your explanation and I can agree with you that working with the primitives is not an easy task to do. Indead you can't set the exact size in the inputfields.
I think it should show a little screen when you choose a primitive on wich you can fill in the exact start values in the three axes for a cube or a radius for a sphere instead of the edge size of a single face and so on.
Maybe it is something for the future?

Anyway thanks again and best regards,
Frits