How to choose the right dimensions of air box using HFSS?

Hello,
I have set up an air box around my antenna and defined it as a radiation boundary. The box is distanced at a bit more than a quarter of the free space wavelength of the cenetr frequency from every side of the antenna (except the feeding side). The trouble is that when I increase the dimensions of the box, the antenna parameters (eg. S11) vary substantially.
Could anyoane please tell me how to choose the right dimensions for the box? Should I notice a convergence in the parameters around some box dimensions?

Thank you.

My first thought is that you may not be converged. Let it run more iterations. You may also need to increase the mesh on and in the airbox.

hfss v11 may tell you if your solution convergence or not

i would suggest using pml boundary for radiation problems, especially where radiation pattern accuracy or field calculations are needed to be accurate. this is suggested by ansoft.

see attachement.

yes, I think it's a convergence problem, i've put a finer mesh and now the variations are barely visible.
Thank you