go with the size that you desire, but take into consideration wut you really want from ur car.
i wanted looks and speed. i had 17" chrome on the car when i bought and i hated the way it looked and it took so much power and torque from the car (gts). ive driven a gts with 15" stock wheels and that thing roared comparing it with mine.
now i finally got 17" motegis DP6. oh boy i just wish i went with 18s instead. i mean i didnt gain any torque by switching to these rims and i realized that i didnt care about speed anymore. and for some reason the 17s i got now look smaller than wut they are.
so my answer to ur question, if you want a bit power go with 18s or else 19s if ur for looks. i mean you wanna look good after paying some money on the frickin wheels.