Head room sometimes depends on the height of the seats as well. Like the 3rd gen Vios. My head is scraping when I'm sitting on the rear seats. But for my Wira, it doesn't happen even when I try to sit as up right as possible.The design is to maximize interior space so by going tall they at least increase head room and boot capacity.
---------- Post added at 11:30 AM ---------- 6 hour anti-bump limit - Previous post was at 11:26 AM ----------
Well in any case, it's a small low powered econo car, so just use small rims not exceeding 15'.
If the car is stock, then it shouldn't exceed 15". But if modded in future, anything larger should be better isn't it?