A 13 cuft should fit. As long as the item is not used in the water you have up to 60 days to refund or exchange it.
8079 Tank Pouch
Holds 20 cu ft Redundant Air System 2 stainless steel snaps Mounts on most Zeagle BCs
If you are doing deeper dives, or dives in an extended overhead environment, then you need that added time that a larger cylinder will give. The Zeagle 20 cu ft Pony Bottle System is a Totally Redundant Breathing System that includes a 20 cu ft Aluminum Cylinder, a Zeagle RaZor Regulator/Valve Combination with a 36' hose and a balanced Envoy 2nd stage. The system weighs roughly 12lbs (empty).
Note: There are MANY different ways to mount the 20 cu ft Pony Bottle System Redundant System to your BC. Every way of mounting extra tanks (valve-up, valve-down, valve-left, valve-right, etc.) has supporters and detractors. If you don't see your favorite method below, be assured that you can mount it in your preferred fashion as well.