Scale length is simply the distance between the bridge of the instrument and the nut. It represents the part of the string that vibrates which is sometimes referred to as Vibrating String Length or VSL. Your total string length is a fairly inconsequential measurement beyond the geometry of string angles approaching the nut and bridge.
On a western chromatic or diatonic fretted instrument, scale length determines fret placement. With the 12th fret position being exactly the half way point between the nut and bridge and one octave higher than the open note. For a complete explanation of intonation on a fretted instrument, please see my article Intonation for Everyone.
The remaining frets above or below the 12th fret are on a graduating scale, and the shorter the scale length the closer the frets are too each other. While there are formulas you can use to mathematically figure out fret placement, it’s much easier to use a computer program. I use a free program called wfret. You can download it here.
Most guitar manufacturers use the same scale length on most of their instruments. Fender uses 25.5″ lentgh where Gibson uses a slightly shorter 25″ scale. Martin uses a more esoteric 25.34″. However, many import makers use metric scales that are close to 25″ mark but my not be exact. An easy way to reverse engineer scale length is to simply measure from the nut to the 12th fret and double that measurement. For example if the measurement is 12.5 from the nut to the 12th fret than the scale is 25″.
When designing an instrument, You should decide the scale length first as it will dictate bridge and fret placement that will be critical in the overall layout of your instrument. To long of a scale and the neck could be heavy and out of balance to the body, to short and you won’t be able to play on the upper frets. If your building a box guitar a yard stick can be handy tool to give you the feel of what scale length you should use with a given size box.