Rob
Forum Moderator

Coefficients are always an odd one: the forces are just read from the pressure/shear results. As it's a coefficient the area you need is based on whatever you're comparing with: ie how did you compute the coefficient from the experiment. However, they are convenient and often used inside Fluent for the particle drag laws. 

More usefully, Google of "drag coefficient formula cones" (lose the quotes) has a decent article from Aerospaceweb.org which seems fairly comphrensive.