John2.5RS - The rear sway bar size is dependant upon personal preferance and ability to drive. If you are skilled, you can drive on a 22mm bar all day long an you won't notice any adverse effects. If you're not so skilled, an 18mm bar will reduce understeer (by causing more oversteer) slightly and even the car out more. Also if you get a suspension on the car, the rear end will be stiffer an may not need such a large rear bar as before.
Like I said, it all depends on your driving style and preference.