A 7 oz trout and a 12 lb salmon might benefit from different knives.
Personally, I no longer fillet using the Scandinavian style (long, thin, perhaps slightly curved, very flexible) fillet knife that most American fisherman like for their tackle boxes. That said, for that type of knife, anywhere from 6" - 8" is considered a good do-it-all length and something longer for fishermen who go for bigger game -- like salmon.
There are all sorts of knives at all sorts of prices. A $200 knife will stay sharp longer and look a lot better (I hope) than a good $15 knife. On the other hand, that $15 knife will do an excellent job as long as you get it and keep it sharp. It would help to have a little more input to narrow down the field.
I think Rapala, MAC, and Knives of Alaska make good sportsman's style knives priced fairly, and according to their quality -- with the edge to Knives of Alaska.