No seriously, 306's are set up for 15" wheels, you can fit up to 17" wheels without messing up the speedo,
the reason why most 306 owners don't use 14's is because of the massive side walls on the tyres and you get unwanted flex on the tyres which can make the car feel more wooly,
15" wheels are by far the best size to go for on a 306, get some 205/50/15 fitted as well
the best compromise between ride and handling, the side walls are just to massive to be performance oriented
it's also what all after market suspension is setup to use with,
why do you think the gti6 and the rallye have 15" wheels?
and they are still noted for there excellent ride quality