Remember that what you are looking for are internal dimensions and not the outside dimensions of the box.
So using internal dimensions, multiply the height times the width, times the depth (in inches).
Then take that number and divide it by 1728. 1728 is 12" times 12" times 12" or 1 cubic foot.
So if your box is (internally) 7" wide, 12" deep, and 40" tall, then it is 3,360 cubic inches or 1.944 cubic feet.
Get it?
Don't forget to use a lot of braces to keep the box strong and free of resonances. You'll also have to allow for the amount of air that the braces take up too.