What have we come to these days. Such a small city econo car wants to put such huge wheels!
Myvi just use maximum 15' lah, that's the best size to improve your grip and handling without sacrificing many other things. Even if you use 16', tire is already very thin 205/45 which is just over 9 cm sidewall height. So your ride comfort will suffer a lot, like riding a tuk-tuk bajaj! With 15', you can still get 7' wide rims so can use 205/50 tires which is 3 cm wider than std already but yet still have >10cm tire sidewall height so still comfortable and still maintain about the same overall diameter. The overall package will also still be light enough for your small econo engine, and you don't need to bust your wallet to find lightweight rims (can still go for normal fake Taiwan rims, no need to get ultralight expensive forged original Japanese rims).
Wait until you have a rather big sedan with over 200HP then can start thinking of using 18' rims! Even me driving such sedan only use 17'!