I tend to prefer smaller units. 1k were doing well enough for me so I hadn't bothered increasing their size. With the artillery, the csa has trouble getting enough of the good cannon, so I use smaller units for a while until I have accumulated enough to expand to a larger size. While my army is still growing I would always prefer two 8 guns units to one 14 gun unit. Once I've maxed out at 4-5 corps then I'll increase beyond 8.
This is only one way to play though, very possible to go about this differently.