James,
It would be 6" to fit in the lid of the top box, with a slight hangover, this was the best solution to enable a stable sealed unit, but the volume of air inside the unit was not enough for a good airspring, and so the Bass produced is affected, plus it plays out the top of the car, so i discounted this. I looked at a box in the boot, but you would need to port the trans tunnel to bulkhead from boot to seats, so discounted this. I then looked at the method in this post, the problem i found was that even if you could create a sealed enclosure the shape and size would again not give the correct volume required. Looking at the video of this one playing the enclosure is not sealed and therefore air spring effect required is not present, you can see this by how the speaker moves, it is not a linear in and out motion, but more of a wave formation, thus producing a poor signal and will ultimately lead to destruction of the speaker. In the way it is being used it is to produce very low bass frequency responses below 60hz (I guess):
view at 1:24
http://www.youtube.com/watch?v=d88h3XzIOmQ you can see the deformation in the speaker as it moves.
Now look at this one
http://www.youtube.com/watch?v=9MjprbYR ... re=related the speaker is moving correctly as it is supported uniformally, sealed unit correct volume of internal air
Now the system i built is not to re-produce just the low frequencies in the sub woofer, but to play the bass frequencies up to 100hz, with the woofer, mids and tweeters taking the frquencies from 80hz upwards, thus giving a balanced feel to the sound, i.e. the sub woofer is always working (modern remastered music plays bass upto this frequency). The speakers in the front of the zed are placed very low in the foot well and face the transmissionn tunnel, this makes it difficult to lift the sound , ideally they should be angled to the back and upwards (requires a bespoke pod). to overcome this, mids and tweeters are used in the doors which lifts the msic up and to the front. This then gives a problem of no fill sound at the rear, and this is the reason for the small rear speakers, which fill the gap in the sound station (I recommend keeping or upgrading do not disconnect). I changed all the speakers to front and rear to 75 RMS 5.25 components, and installed a slim active sub, upgraded tweeters in the doors allowed for the increase in ampage, then it was a case of tuning the speakers for LP / HP and frequency over lap, front back bias, Bass & Treble, this provides a very deep crisp musical environment, hood up or down at the lights or driving at 70mph.
As you alluded, there are two different systems here, and neither is wrong, it depends on taste, It is good that other forum members can review both and choose the best way forward for their listening preferences, I am sure they can all be improved, there is some decent kit out there, your just constrained by your budget. One thing I think is a must is HU you can change the filters and frequencies on, I would not of achieved the sound i have now with the old HU that came in the car.
Boysie
I am like you, I have spent a fair bit on my HI Fi at home favouring an ONKYO Digital Integrated amp and a solid pair of bi-wired speakers on good solid stands for music. Then i Have my surround sound system on my 50" DLP, this is self built to to a THX quality receiver ported sub, tannoy speakers etc for movies. I have two systems because the sound requirement and quality is completeley different for each application, just like the two car builds here.
I like the home Hi-Fi especially on classical music where you can here the strings on the instruments, brushes on the symbols etc, something you will never get in a car.
i have a feeling maybe 6" which isn't a lot of room to play with.