Let's say: 12 x 12 x 12 for an 8 inch.
24 inch for two eights, add another 20 inch for a horn, ribbon, or tweeter, maybe a midrange as well. Or, omit the midrange and run three eights GT Audio style. Tower speakers that gets your footin the door, and more sensible for the average apartment, condo, or small home.
Obviously, the amplification and processing needs to be scaled down.