There is no "correct" size for meshing. It depends on the resolution and accuracy you need for your application. Decreasing the mesh size generally increases the accuracy of the output parameter.
If you are getting grossly different values from one meshing size to another, you might want to look at your model and your meshing shape relative to the geometry of the section you are meshing and to the parameter you are assessing (principal stress, von Mises, etc.)