: I'm not sure what kind of volume is recommended for airbags.

"Full."

A good reason to make your own custom bags to the dimensions of your boat. Use Coverene or equivalent PVC with HH-66 cement, or heat-sealable nylon. Tom Yost's version worked for me (link below).

It doesn't have to be a perfect fit, if you don't mind the weight make it oversize. Locate the tube so it doesn't kink when you inflate the bags. When you carry gear put it under the bag before inflating, so your ballast stays in the keel in a (near)-capsize.