Subwoofer Enclosure

Depends on the sub and the type of enclosure.  Performance of the sub itself will be affected by the size.  Resonating frequency will change with the volume or size of the enclosure.  Any decent sub manufacturer will include product literature which details this information for you to customize the sound. 

Usually the specs from the manufacture are for the best overall performance of the sub.  As F4Pilot stated, you can change the size of the box to change the response.  A common, easy way to do this is put insulation inside the enclosure.  It changes the resonating frequency and, if I'm not mistaken, makes the sub perform as if it's in a larger box than it really is.  If you want to go smaller, but don't want to rebuild the enclosure, then add some extra MDF inside the enclusure to reduce available airspace (make sure to seal around the new wood to keep it airtight).