How to select the outer face of a part

Hi, I am working on a macro to extrude a sketch and I need to apply it on an outer face of a part. 

I was wondering if that is possible via a macro to detect the outer face of a part (which should be also the largest face, so 2 conditions) and if yes how and which parameters to take into consideration.

 

Any documentation, tip or help would be very appreciated.