I did a five gallon wiezen batch not too long ago with corriander and orange peel. I ran the corriander through my grain mill which worked great. Used half an oz. For the orange, I used a hand held kitchen kitchen grater. I used all the zest from one whole orange. Just stood over the brewpot and grated it directly in. I added both for the last 15 mins of the boil. Seemed like just the right amount of both the corriander and the orange to me in the finished product.
One thing I did notice, is the orange flavor became more pronounced over time in the bottle.
Another thing that makes a world of difference in wheat beers is the yeast strain. I really prefered the hefewiezen from white labs.