Its all about the size of the Sub Box, the sub can be tuned with different sized enclosures. You cant just make one just any size if you want the best out of your sub.

depending on what sort of sound you are after a 6 or 8 inch in the right enclosure can sound really good too.

I have a good mate who does all this stuff for a living and I could help you with the size of an enclosure if you know what sub you are going to use.