Mesh size and dimension of the structure

Hi Guys,

I am quite new in COMSOL, and I have three small questions about the setting of the mesh in ewfd, frequency domain study.

1. I found the results will be different if I used different size of the mesh, in this case, how could I value the setting of the mesh is suitable or not?

2. Another problem, the different dimension of my waveguide with same mesh size will also yield different results, how could I determine the right size of mesh if I change the length of my waveguide?

3. The distance between the light source and the boundary to my waveguide will also get different results, how could I choose the distance of the boundary to my waveguide.

Is there anyone have good suggestions? it would be very appreciated.

Chuan