Tyre shop of course wants to take as much money as they can so never really trust what they suggest! For Myvi just go for 15 cuz it's only a very small city car with small engine & low power. If you use 16 or bigger, your car will get even slower and more over it will consume more fuel and make your braking longer. Get the lightest 15 you can afford (a good one should be around 5kg per wheel) and at least those made in Taiwan if you cannot afford original Japan ones.