Problems with mesh definition

Hi,

I want to know if it is normal results change with the size of the mesh?

I am using RF module, and implemented one of Comsol examples the Branch-line Coupler, when I change the size ​​of the mesh (Course, Normal, Fine, Finer, etc) the final results (computed values) keeps constant.

But when I try to implement a simple microstrip line, and change the size of the mesh the final results also change.
It's normal? If not, where's my mistake? How do I find the optimal size of my Mesh?

Greats,
Daniel Santos