17's with say 215/40R17 tires is almost exactly equal to the stock rims and tires you have now. You can go 18 with the stock setup too no problems. When you lower your G is when you need to start considering size and offset.
Basically you can go as big as you deem possible. 20's could fit hypothetically, but you'd need a nice fender roll for that, but that would just look dumb IMO LOL
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