Depends where the battery is located. If a four seater with the battery under the bonnet and only a relatively short cable run to the solenoid you could probably get away with 16mm2
If its a 2 str with a battery down the back and a long cable run, I'd go for 25 mm2 to minimise voltage drop.
Having said that, I've wired my Plus 4 4 str with 25mm2 but that's because I happened to have a length of cable spare.
I also used proper crimped on terminals and bought the special crimping tool to put them on, think it was less than 10 quid and produces a nice strong crimp. Use the proper insulating boots as well for a nice neat job.
Arwyn
