Kharon
Contributor
... but I would suggest anything smaller then the 13 is worthless and here is why I say that: when you go to your redundant air supply, there is a reason why you are doing that. That "reason" is going to cause you some anxiety, the larger redundant air supply will give you the confidence to slow down, problem solve, and make your exit without you feeling rushed.
That certainly depends on your diving profile, skill, and confidence. Certainly if you are staying above 60 feet and are not prone to panic, a 6cf is plenty. I always traveled with a 13cf and it was doable. I found that for my diving style and skill set a 6cf was fine and a whole lot more transportable. Not everyone needs 20 minutes of independent air.