You need to get the Thiel-Small parameters of the drivers (will be in the subs spec) and then you can calculate the size of the enclosure.
If the above is doiuble dutch seek pro advice!
Then you need to decide whether you want a sealed or ported box (sealed box = tight natural deep bass, higher power handling but not as loud or a ported box = "BIG" bass, rolls off quickly below port tuning freq and needs more careful positioning/tuning as you are adding another variable). It depends on what kind of bass you require, what will fit and where as to what enclosure will suit your needs best. A box thats designed for say a Fiesta will not neccessarly sound nice in Escort.
Personally I prefer sealed enclosures.