This particular neck is designed as a 23" tuning length with 19 frets. This is determined by using an online calculator on the stewmac.com website. There are calculators for string spacings and string height info is on various sites. Then it gets more complicated. The choices of fret widths and heights, string sizes, and hardware to use can be mind boggling. Fortunately, cigar box guitars shatter all the rules and makes it easy. Just use what is called "found hardware". Go buy a book called An Obsession with Cigar Box Guitars
by David Sutton and you will be hooked.
The box you use will determine how long the neck material you use, due to the length of the box. These two are about 9.5" long, so I have two files that are identical other than at the box end and the added thickness of the fret board.
to play might be your life challenge.