The ground size of antenna in Active integrated antenna

Hi
I want to ask about ground size of antenna when designing antenna in active integrated antenna (AIA).
Because the S parameter changes then the ground size change so:
When you simulation and fabrication antenna(ony antenna part) for AIA, which ground size do you choose. The ground size is equal to the whole circuit (icluding antenna and microstrip circuit) , or any ground size is Ok to measure.

Thank you for reading my topic