supports for overhang

Hi, I'm new to the forum and need some advice.

I've just had granite counter tops installed. It's 1-7/32 in. thick.  It has a 20 in. overhang.  the overhang is actually a curve of about 120 degrees over a standard rectangular cabinet (approx. 24 in. wide).  The 20 in. overhang runs a distance of about 50 in. on one side and 40 in. on the end. (Basically a big ellipse hanging off one side and the end). 
We plan to use corbels to support it.  My question is what size and spaced how far apart?
The installer recommended 2 six in. corbels on the corner, one to support the side span and one to support the end span.

 
Any info would be greatly appreciated.

Thanks Mike