Henderson's Chicken Chart has some stats on laying and egg size.
According to their tests Delawares laid the largest egg. I have had many different breeds and my biggest eggs came from an old Austrlorp hen. My black giants laid a medium egg. Nor even worth considering for size. I knew a guy that raised Turkens and I was amazed at the consistent size of their eggs.
Of my current flock, the Welsummers not only lay the biggest eggs but also the most.