Most likely in the rear it will be a 30 series sidewall tire. 40 and 35 are too tall and those will rub regardless of how much wider they make the fenders. The fronts fenders only seem widened since they have much less clearance than the rears. "old" model S' can fit a 285/30/21 with the correct offset rear tire.If I had to guess for staggered sizes:
Front: 255/40-19 or 265/35-21 (we can zoom in on the tire pic and see these.)
Rear: 285/40-19 or 295/35-21 (guessing)
Source? Playing around with sizes at Tire Rack. Both 19" sizes are available in Pirelli P-Zero; both 21" sizes available in Michelin Pilot Sport 4S.