Not sure size really matters here James :-)
In my view, larger boxes are easier to hold, so I prefer to use them if I can get them.
I think the internal volume of the box is more important, and how you attach the neck.
With straight through necks I reduce the size going through the box as much as possible to maximise the volume in the box.
Also having as little of the neck as possible coming into contact with the top & bottom of the box gives "fuller" sound.
The material that the box is made of is also important in getting good sound. I particularly like Oliva boxes for this reason.
Shanes Field Holler guitars are around 7 X 9 X 2 and produce great sound.
You can control tone to some extent by using different gauge strings.
Size & placement of sound holes will also affect projection.
At the end of the day I think you are still going to have to experiment to some extent to get the sound YOU want.
Try watching people playing 3 string CBG's on YouTube. You can see what box they are using and hear how they sound.
Sorry if I've further added to your delima :-(
I'm now trying big distributors... I cant believe its this hard. I mean, almost not worth bothering any more.
I wanted to build more than paper covered things, you know?
The gems on ebay either won't reply, or stop replying when ready to buy a bunch, or wont ship to canada or want an extra premium for all the terrible hassle.
Did I say, buggers ?
I've had quite a few boxes sent to Australia from this supplier.
Other than that, why not build your own boxes?
Been there Bruce.... thanks mate
I think I might try making my own. More I look seems maybe for something the size of a strummer and even a guitar if only 3 string, the difference in sizes wont make a huge difference.