Scale Length is simply the length of the guitar, between Nut and Bridge.
Most modern Classical or Classic Guitars have a scale length of 650 mm, but this isn't fixed. The scale length can vary between mm for a Señorita guitar, right up to 664 mm for some of the larger Concert Guitars.
Each Guitar Orchestra Instrument has a different scale length.
Within the Niibori family, the most common scale lengths are :
Alto guitar : 535 mm
Bass guitar : 700 mm
Contrabass guitar : 750 mm
You'll find more information on the Hampshire Guitar Orchestra (HAGO) website