Pierre-Yves,
Scale is the length of the string that vibrates at the desired pitch, so between the nut and the bridge. Like Bill says, the easiest way to measure is to take the distance from the nut to the 12th fret, and then multiply by 2.
In centimeters:
medium 32 = 81.3 cm (12th fret at 40.65 cm)
short 30.75 = 78.1 cm (12th fret at 39.05 cm)
You won't find many SC models with a scale over 30.75, because of head-dive problems.
On most Alembics, you'll need slightly longer strings because of the separate tail piece.