If your existing speakers came with a tech sheet in the box, usually you will have an ideal number to go by airspace wise.... BTW cabinet design is universal... all the principals are the same. The only difference is the way a speaker will react. That is why they give you all those nifty numbers they call "theil-small parameters"

Anyways if they give you an idea box size like 1.25 cu.ft. per driver, you can use the formula LxWxH/1728 to get cu.ft. Be sure to use INNER dimensions to get your airspace and not OUTER. an inch and a half all the way around can make a large difference in how the driver reacts.