It definitely depends on your outlook towards vehicle style. traditionally, I try to +1 or +2 a vehicle, and set it about .75"-1.5" dropped. This car already has a nice drop (enough to scrape the front bumper sometimes as-is), but it could sustain a tiny bit more. Given this, an 18" wheel is already nice and big, but a 19 is not out of reason. Especially if the wheel has a lot of void space, like a thin spoke wheel would. Reason? We have some great looking wheels.
I'm looking to into Petrol wheels, TenzoR (a certain wheel they have looks decent), a certain Rota, TSW, and ADR. There are a few thin 5 or twin spoke 5's that I find a nice look. The Petrol wheels, if they fit, would be AMAZING in 19's, IMO. They use a stainless steel wheel lip on the outer edge, and it looks like a 2 or 3 piece wheel this way.