Danco Sports fishing fillet knives run from 4" to 9", with blades from highly flexible to nearly stiff, so you can match the knife to the fish on your cleaning table.
Which length? A 4" to 6" blade suits panfish, trout and small bass. A 7" blade covers most inshore fish like redfish, seatrout and snapper. Go 9" for bigger fish such as mahi, kingfish and grouper.
Tournament Series, Pro Series and ECO-1 blades are German 4116 stainless steel; Lunar-1 blades are Swedish Sandvik 14C28N. Not sure where to start? Try the Knife Finder, or shop boning knives and serrated fillet knives.