Call you building inspector and ask him what code is for that size footing. I just built a single flue chimney, the block is 16"x16" and code was for a 24"x24" pad(footing) also make sure you're at least 4' below grade for frost.I went down 6' and made the footing 30x30, I tend to overdue things sometimes
. I would build a masonary chimney before instaling a SS, masonary will out last you and the house where the SS will last about 15-20 years. I priced out both and the SS was actually more $.