You get the desired volume before you build the enclosure. That's involved in the design, not the construction.
The easiest way is to use Sketchup to build the enclosure in a 3D environment and then leave the top off. From there, select the section of the bottom and get the area from it. Multiply that number by the height to get the total volume of the enclosure. If the height is in inches, divide it by 12 to get the cubic feet of the enclosure.