I voted 15", would be ideal for me, I like much rubber in profile.
16" would be fine.
17" would be marginal for predictable handling.
18" would be

.
19" would be

and probably would make me to not buy this car.
I believe 205/50 R16 all around is perfect for this car.
If you look at some current cars you can make reasonable comparisons.
BMW 130i is a quite heavy RWD with 205/50 R17 tyres all round. It can barely spin to drift its tyres in 2nd gear and it has a 3 litre inline six engine with 265 hp and 30+ kgm torque. Quite snappy on its reactions and not so much predictable.
Well, by now you will be wondering who am I to judge things. I am just a petrolhead as many of you are and also had been an automotive journalist for 10 years in car magazines (EVO, Top Gear and others) and I have driven about 700 different cars rangning from Ferraris and Lambo's to Fiat 500's and Toyota Starlet's. Trust me, for driving fun you don't want big wheels in a responsive RWD lightweight sportscar...