In 18", 18x11 up front, and I think Vorshlag got 18x12's to fit out back. With either 18x11's all around or the 11/12 stagger and no spacers, you'll end up with significantly different front vs rear offsets, maybe more than 15 mm, and front to rear rotation will not be possible.
Up front, there isn't room for any more than 11 without getting into custom bodywork. In the following picture taken just after fitting the 18x11's there is barely 0.045" wheel or tire clearance to the strut with the camber shown (about -2°), and on one side of the car it took an 0.025" DIY-fabbed spacer to get that much. With 19" or 20" wheels you'd gain small amounts of inside clearance, but not nearly enough to allow even 11.5" wide wheels without at least another quarter inch showing up outside the sheetmetal.
Norm