Small or large sub for music

Size and # drivers (including passive radiators) is often a general indicator of how low a sub might go but you really have to check the specs to get a good idea how a sub will perform

Also lower bass frequencies take exponentially more power to produce flat response so you better have plenty of good clean amp power in those subs too to carry the load. The more the better. Not enough power in reserve is perhaps the most common cause of bad sounding bass.

Is this so you can enjoy accurate bass while doing your yoga
headstand against the wall?

I don't need a wall for that.

Ideally I would want  2-4 larger quality subs distributed around a room but practically I do not want to have to deal with all that so I go with just a single smaller sub that is easier to manage.   This is in a second system.  My main system has full range speakers so no subs needed. 

noble yes for 20 hz in large room you need at least 1 larger sub up to teh task (check specs) or multiple subs or drivers. Distributed multiple subs or sub array allows more  for smoother bass response throughout a room than extending low end frequency response..

Room size and low frequency extension desired determines how large a sub need be. In general you need a larger sub with a larger bass driver or possibly somewhat smaller if multiple drivers or passive radiators are used if your room is larger and you want bass to extend all the way down to 20 hz or so which is typically about as low as any music like low pipe organ notes might go.

So basically size does matter when it comes to extended bass.  

For example I use a 8" sub with dual pasive radiators that can go down to almost 30 hz or so in a modest sized room.   I use these with small monitors somewhat similar in size to kef ls50  that do little below 60hz or so.    I don't know of any any single driver 8" sub that can do that.    

If your mains are larger and more full range you will likely have to go with a larger 12" or even larger sub iin order to cover the lowest frequencies not covered otherwise by the mains.