The Neuspeed front bar is 29mm (others may be also) and is the same as the Audi Sport Suspension Package Option. If you have a sport suspension, then you do not need this part.
Check the Audi dealership for cost of a replacement vs. the aftermarket cost (Neuspeed is about $260) and buy whichever is cheaper since the Audi dealership may not charge you shipping and they are the same.
For a rear bar, it really depends on what you want to do. Two common options are (including with Neuspeed) a 25mm and a 29mm bar. For normal sporty driving on the street the 25mm bar will still retain the understeer common on most US cars, while the 29mm bar may allow the back end to come around on you if you try.
Good luck, both Neuspeed and H&R are good bars with Neuspeed being a bit easier to find. Here's the link to Neuspeed's site:
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