Hi, here is a really good discussion environment but more discussion make more questions indeed! 

Now I am confused about mesh size and made the following simulation example to analyze:

a beam model dimension : 100mmx100mmx500mm

Force applied at the end :  5000N

Study : static study

Fixture : one end of beam

Simulation result:

Default mesh density = 17mm    Max. stress = 15MPa

Fine mesh density = 8.5mm   Max. stress = 18.6MPa

So, I increase the beam length to 2000mm

Default mesh density =27mm   Max. stress = 57.0MPa

Fine mesh density = 13.5mm   Max.stress = 68.4MPa

This make me think about one question: Should I use the fine mesh density(8.5mm) for the 2000mm beam simulation?

If I apply 8.5mm mesh density, the max.stress will be  78.0MPa

So, in solidworks simulation, how to choose the mesh size for your model? Should I consider the model volume?