I think you're both wrong, Jack & Niel,
size doesn't affect Cd at all. That's the whole point. Same shape = same Cd. A 1x1x1 cube in theory has exactly the same Cd as a 2x2x2 cube. The frontal area of the 2x2x2 cube is 4 times larger so the total drag is 4 times larger, but the Cd is the same.
Check out the link below for more info.
https://ecourses.ou.edu/cgi-bin/eboo....1&page=theory
Note how different shapes are given different Cd's, with not mention of thier size.