Well, usually, what you want to do. Is First, grab you're max dimensions, now, when doing this, you want to make sure you keep in my mind for other things. Such as the amplifier(s), battery(s), wires, etc. Once you find these figures, you will do the math, and see how much gross space you have. Then from there, you will calcute you're net, but subtracting the width of the material you are using, bracing, and also, you want to revisit this or take into consideration of the subwoofer(s) displacement. So then finally, you chose what and how many drivers you want to use, but you cannot forget about port displacement if you are going ported. I hope this helps.