You need 19-20 inches. Smaller are too frustrating. There are essentially three sets of ribs. Two sets are ill defined and free-floating and made up of finer bones than the main ribs which are of coarse stated to the ribs.
Start by filleting it the normal way as with any fish....over the main ribs and off at the tail. Clean up so it is not slippery. I usually have a 1/2 paper towel under a fillet.
Take your fingers and finger nails and feel around on the fillet to find some protruding bones. They're outside of the main ribs and the third set is between the tail and the end of the main ribs. They face one way or another and I always forget which way that is until I trace around a little with the knife. Before long you will learn to carefully but quickly slice down each side and remove these nasty little bones. I prefer to use a Victorinox Farmer or similar. It has a stiff, thin, narrow blade and very quickly gives you confidence in your work. Must be razor sharp. These knives are particularly good at the initial breaking down of the fish. I only use the fillet knife to skin fish. Throw the fillets into salted water and chill. Once chilled rinse of and freeze in baggies and enough water to cover them.
Theyre every bit as good as Perch and shares that type of firmness and texture. My wife, whom is very picky, Likes them lightly breaded. I hope you enjoy them as much as us.