If you browse the Realistic Dildos section of LoveHoney, the products are shown in order of popularity... Discounting the ones that would have sold a lot because they're on special offer or in the sale, you can see that 6-inchers out-sells the 8-incher...
http://www.lovehoney.co.uk/store.cfm?cat=16
But then, the Fat Man Mega Dong has sold out! http://www.lovehoney.co.uk/product.cfm?p=2473