The drag coefficient is quite a function of Reynolds, and potentially other factors. Man, I must be in a good mood today. Let's see what I can find. For REALLY low Reynolds numbers, (Re < 1), we haveThe drag equation ( propertional Area * velocity^2) is an approximation for high Reynolds number flow (eg air) it isn't necessarily correct for low speed or high viscosity cases.
The area you use is the frontal area, which is pi r^2 or (pi/4) d^2.Also from the drag equation, where I use area, will it be the surface area of the sphere, i.e. d^2*pi.
Thanks