40mm is the 'standard' drop. It vastly improves road holding and reduces body roll.
It depends one what wheels you have really, have a look round the Cinquecento / Seicento section (where this should've been posted really ). Theres heaps of info in there.
Pi springs tend to drop lower than they say on the box and result in a rather harse ride.. avoid them imo
40mm is about as low as I'd go on 13's, any lower and speed bumps can become intimately acquainted with your undercarriage, which can be painful. Apex drop around 40mm and aren't too expensive.
yeah the a 40mm drop will be fine on the 14" .. apex or spax are good reasonably priced springs although if you go for 40mm its a good idea to get a full kit with shocks too as the original shocks wont last long on 40mm ... i think 30mm is the limit for standard shocks