[sswee]

Just to reiterate what luminous said.
A CD is chosen for speed (run time) and pore size.
To choose what CD will work best for you, you'll have to consider several things.
The first thing to look at is the PS capability. From Fibergeek's experiments, you'll need 2.5x the CD plus 2V to 5V for a max voltage. I've used this equation since I started and found it to be valid. IE. to run a 6A CD, you would need 15V minimum 18V preferred available from the PS.
The next thing to consider is the tank size and the size of part. The higher the CD or SA, the more heat generated. If the tank cannot absorb and distribute the heat generated by a higher CD or SA, you'll combat dissolution. Tank temp is directly correlated to voltage demand. To push a set amount of current takes more voltage at colder temps than it does at the warmer temps.
What worked for me was to start out at a lower CD and work my way up a little at a time. I made notes on the variables so I could track which changes had what effects. IE. How much a change in CD effected tank temp with the other variables the same, the tank temp effect when CD, SA and other variables were the same. It helps to run similar parts and only make one change at a time so you can tell what is causing what.
Keep at it and it'll all fall into place.
SS