it's not so much the size but the weight. Below ~24 lbs. and you won't really feel a difference over the OEM 18"s. Slap on some 30+ lbs. boat anchors and everything suffers.. acceleration, fuel economy, rattles, lighter wallet from the sneaky tire mark up for that extra inch.
And aesthetically, the 986 (imo) looks best with wheels where you can't see clear across to the rotor.