Realisitcally? Most just fit the biggest they can in, or have some preconceived notion about what size sub they need based on the 'perfect' size (usually 10's or 12's for those guys).
People who know what they are doing size ther subs according to what they need to fill the gap in the rest of their system. This means level matching, frequency matching, etc. For example, make a poll, see how many of the guys here running 18" subs have 6.5's or smaller as the next biggest sized speaker in their system. Or, watch the next several threads you see where people ask what sized subs to get... almost none will have someone responding asking anything about the rest of the system they should be trying to blend with, its not even considered.
Balanced systems are few and far between these days. No, speaker size choice these days is based mostly on voodoo, rumors, bragging rights and simple aesthetics. Pretty sad really.