2x1.6 front and 2x1.7 rear is pretty much the standard starting point on pistons.
If you are rubbing the bulkhead with the servo arm, take the arm off the servo and re-orient it 1-2 teeth more towards the left of the car on the servo itself. It is hard to tell in the manual (stare at the illustration on p20 for a while and you'll get it) but the servo horn doesn't actually go totally upright for center like most cars.