Specifically for a hefewiezen, you really don't need any specialty grains. A good quality wheat malt extract is all you need.
In general, though, there are guidelines for specialty grain quantities, depending on the style you are trying to make. I usually look at several different recipes before settling on quantities. Also, Ray Daniels' book Designing Great Beers is an excellent resource for formulating recipes.
Also, it's easy to go overboard with specialty grains-- restraint is usually called for, at least in my experience.
On Deck: Jamil's Vanilla Robust Porter
Fermenting: Orange Blossom Mead
Kegs: Element 56 Pale Ale, Ron's Belgian Blonde, Summer'n Saison, Furloughktoberfest '09, Grateful Pale Ale, Sam Adams Cream Stout Clone, EdWort's Apfelwein