Scale Length
The scale length of a guitar is defined as the distance between the nut and the saddle (before taking into account any saddle set back to help intonation).

650mm is now regarded as the ‘standard’ scale length for modern classical guitars.

Some famous makers, e.g. Ramirez, have moved to a 660mm scale, but this can make the instrument more difficult to play.
Going the other way, guitars with shorter scales, such as 640mm can be easier to manage for players with smaller hands.

Non-standard scale lengths can usually be made to special commission, but it is perhaps worth bearing in mind that they might command a lower resale value.
Neck width and string spacing

The width of the neck at the nut largely determines the string spacing, and hence can affect ease of playing. It is usually between 50mm and 54mm.

On classical guitars, the strings are spaced equally, My spacing for a 52mm nut is 8.4mm (which gives a 5mm clearance on each side of the fretboard).

String spacing at the bridge is 11.8mm.

These measurements, along with the shape of the neck can be modified on commissioned instruments. Please enquire for more details.