setting the air layer

Hi,
I am doing something about Patch Antenna with HFSS! everything is OK , but I do not know how to set the range of the air. somebody told me that it should be longer than 1/4 from the edge. Is that right?
Is there some way better to design the air ?

Secondly, how to design the groundplane ?
whether it need be in accordance with the actual situation?

thank you very much !