It is really not that important how the subs are positioned. There is no special benefit from them if they are turned upside down - everybody would do that, if there was some advantage! It is just one of the possibilities how to mount the sub - in many home sound systems there are so called "down-firing" subs, which are turned towards the floor and make the floor shake more - maybe it's a good feeling when watching a movie.
And, yes, the size of the box MATTERS!! In fact, almost everything depends on the size of the box. If you're planning to build a sealed enclosure than the volume is not so much important as it is with ported (and bandpass) systems. When building a ported box, pay attention to the size and the lenght of the port also.
To find out what size of the box you need, you have to now exactly what sub will you be running and all the technical data about it, put it in a box desig program (i.e. WinISD) and it will calculate it for you. If you have no experience with that, maybe some more experienced member will calculate it for you.
- Pioneer DEH-5500MP
- Magnat Xtrema 216 components
- macAudio aluminium 12" sub
- Phase Linear 1.2kW amp