Your box is somewhat small but not unusable. The length you are asking about really depends on what you are making. First off -- there are no rules. You will get a ton of opinions and directions here but it really comes down to what you want to make. So I would say, decide what you want to make (simple CBG, CB Uke, CB Banjo...). If you think your box is small, try a Uke. I don't make them but there are bunches here that do.
The challenge you are going to face is the volume that you get. Of course, a pickup (piezo) would help.
I think from reading your post that you're worried that if you keep a longer scale neck, you'll end up with 1.2 million frets before you get to the edge of the box. You don't have to put them all the way up. Just do however many you think you'll need (then add 2 to be safe!).
I've used box's not much bigger around 7 3/4 I think. I used a 24" scale (mine are fretless) and the proportions look fine. No rules and it's a guitar made from a cigar box so don't worry about it just build it.