Ok, so let me say first that it really doesn't matter. A cigar box is a cigar box. Not a guitar. Its not meant to be an instrument, HOWEVER [grin], many prefer one 'brand' over another. I myself like Macanudo Portofino Cafe for acoustic type CBGs but have many different boxes and each really will make a nice sound if built well. Some say deeper, some say wider, some say longer. I gotta tell you, it really depends on how you build and by that I mean, the scale, neck, frets or slide, how you put the neck on, so forth.
Here's what you do. If you have a choice, go for a nice sized one [and that is up to you]. If not, build with what you can get. Simple, simple.