Scale length doesn't depend on the box, but how you want the CBG to sound, and by that I mean how long your want the scale to be. The story goes that a Fender Strat has about a 25½ inch scale, and a Gibson LesPaul has about a 24¾ inch scale. I go with a Paul Reed Smith scale myself at 25 inches (warm tone with a lot of room for room for options).
The number of frets is up to you as well. Are you planning on mainly playing it with a slide? Will the action be good enough for notes past the 14th fret to even resonate enough to sound good? Will you even chord it past the 12th fret.
I am very much a newbie, but your question comes with several others for me. Hopefully I have been a little help and not a discouragement. Best advice I continually get on here is to not forget... "It's just a stick in a box". Don't be afraid to fail, or else you won't ever try anything new.
Best of luck!
The scale length is to a large extent a matter of personal taste. I prefer shorter scale lengths in the range of 18" to 22", so most of my builds range on the short side. If you have no preference, and want to make the neck more or less proportional to the length of your cigar box, then I can give you the way I calculate the neck and scale length for a given box. My method assumes all the frets will be over the neck, but this need not be a limit as some builders extend the fretboard over this sound box. I find it simpler to limit my fretboard to the length of the neck outside the box (and some will say this is good because it allows you to open the box should you need to assuming your instrument design allows you the capability of opening the box... not all do). Anyway, here's how I reckon it...
Scale length is the distance between the bridge and the nut (or fret 0, if you use a fret 0). So, the first thing to decide is where you want your bridge. On a non-resonator instrument, I generally place the bridge about 1/3 of the way up the box from the tail end of the box. The tail end of the box is the side where the tail piece will be secured. This allows 2/3 of the top of the sound box to be used by your picking/strumming hand. This 2/3s of the box length will also be part of your scale length. The neck length (exclusive of the headstock length) will be the Scale Length minus two thirds the Box Length. It's also wise to add another inch or so as a fudge factor as little things like the need for space between the last fret on the fret board and the end of the fretboard can affect your neck length calculations.
If you want to double check to see if your neck (or fretboard) will be long enough to hold the number of frets you are planning to have, use your favorite fret calculator (either on-line or separate program) and specify the desired scale length and number of frets. It should produce a table of values with measurements from the nut to each of the frets. Check this distance between the nut and your last fret. Add a couple of cm for a fudge factor and you should have a good idea whether your neck (or fretboard) will be long enough to hold the frets you are wanting. If you decide to go with a short scale length, you will find that the space between frets quickly become too close together on the final several frets, so you may decide to omit them. On full size guitars they often have just 18 or 20 frets, but with a shorter CBG sound box, there is no reason why you can't put a full complement of 24 frets on it, unless your scale length is particularly short as I mentioned before. I usually put "24" into the fret calculator and see what it gives me, then decide it I want to forgo a few frets because the fret-to-fret spacing is much less a centimeter.
Here's a diagram to help you see what I'm talking about...
Hope this helps.
will certainly help.