relation between size of an object and mesh size

  hi all

            i want  to understand the  relation between size of an object and size of mesh ( between  coarse or fine) because ,on a recent -static analysis

of a solid body that had some  structral members ,with the finer mesh size the result was unrealistic , with the default mesh size  the result was acceptable close to therotical calculations  .I would like to know how to select the appropriate mesh size for an object ?

SolidworksSimulation