I know there's an element of 'no rules' when building these things, but being realistic, what would be the longest neck you could feasibly have on a CBG?
And while I'm at it, what sort of length do you think is optimal? I read somewhere that 3ft should get you through the box, a full fret board and headstock. Plus I know you can have much smaller ones...is there any significant difference except the range available>
There are some rules in this area, and it has to do with scale length(positioning of the fret marks) they determine the neck length plus or minus a couple inches. A scale of 24", 24.5", 25", 25.5", are typical scale lengths, some longer, some shorter, but for explanation purposes, lets go with one of those. My current build I went with a 25.5" scale, add 6" for creating my headstock, next, box length being used, will determine where on the box the bridge will be. The bridge will be 25.5" from the nut at the end of the neck(the nut) to the bridge/saddle.
Depending on whether you want your neck to go through the box to create it's own tailpiece, or end inside the end of the box, or bolt on version where it quits somewhere near the end where it enters the box is the next determining factor.
Now lets simplify all of this, 6" of headstock, plus lets say you want the neck to enter the box around the 17th fret, that would be at almost exactly 16"(calculated with fret calculator --> fret calc) Now lets say you have a 10" box, plus you want a through neck to protrude another 3". Add all that up, and you have 35", this is why 36" is a good all around average, I usually cut a 42" piece of wood, allows for some cutting and shaping of my headstock, shaping of the tailpiece, and the extra scrap is used for bracing pieces, so no real waste.
Using one of the fore mentioned scales allows for use of most any standard set of guitar strings.
If you want a longer looking neck, you can have the neck not enter the box till the 21st fret, or make one with 24 frets, but remember, the one rule you have to adhere to for you git to sound right, whatever your neck length, will determine the bridge placement on the box, with the example above, your bridge/saddle will be 9-1/2" from the end or 1/2" from the other end putting it at the end of the box, this gives you a lot of room for pickups if desired, sound holes, large strumming area, as you lengthen the neck, the bridge placement will move towards the center of the box, this is a fixed spacing based on your scale of choice.
Sorry so lengthy, hope this helps, ask any and all questions if still confused.
That is incredibly helpful, thanks!
Good, I was worried I got a little wind-bagged, don't hesitate to ask anything further.
Another factor to add into the equation is that you will get better acoustic volume if the bridge is about a third of the way up the box rather than at the edge of the box.