12" Subs are the most versatile.. and will produce the best overall slope.... but you can make any sub with in reason.. only thing size really makes a difference on is the size of the actual wave created by the sub at any given Frequency... (Amount of air displaced )
Larger subs will create the same wave, just bigger.. it will also take farther for the wave to travel to complete the same wave being it is bigger, and to the ear might not be heard the same in car as it does outside the car.. this is also why bass will sometimes sound louder and harder with the windows down,, because the wave is able to complete it self fully.. you hear the sound passing by your ear.. vs canceling out or not completing fully..
That is why 15's and 18's are best suited in larger cars and vans and such, where 12's and 10's of equal power better suited for the smaller trucks and cars..
There is so much to take in account here.. but the main focus is the box.. in combination with the equipment.. staging and loading the driver..
