First If you plan your dive you dont need one, and should never use one, but since you asked so nice. I use a 19cu and hang a 63 over side at about 20 feet, on deeper dives as well as wrecks. I hate to admit that scince I am a conservitive diver, but we all tend to push the limits. Don’t waste the money on anything less then 19cu. especially spareair units they are good for pool maybe.
I also plan the use of the pony for one reason to pass it to another diver that needs it, not for me. and have never used it yet. I also like to have it in front and easy to disconnect from me to pass it off. I am also proud to say that my vip expired 6 months before I had to fill it and that was just because of a pool dive to burn of air, it took 11 minutes at 10 feet.