The problem with aftermarket mirrors is the small number that include map lights. Since the Sky has no dome light those are important if you want interior lighting at night. You are correct that the OnStar in the Sky is no longer functional, so lighting is the only real consideration.
I am not aware of anyone finding a mirror with lights that is smaller than stock, but there may be one out there.
I am not aware of anyone finding a mirror with lights that is smaller than stock, but there may be one out there.