I make a hidden tang design that is called the “Punta Chivato” and a full tang design that is called the "Shelter Cove". Both of these are furnished in 7, 8 and 9 inch blade lengths. Seven inch and shorter blade full tang designs are called the “Tunitus Creek”.
All fillet blades are made from 0.10-.125 stock. Most popular for large saltwater fish is the 8 to 9 inch blade. A 6 to 7 inch blade works out well for fresh water fishing and as a boning knife for hunting or utility use in the kitchen.
On all designs the blade is narrow at the tip to allow the knife to be slipped behind the gill cover on a fish easily. The narrow section also provides flexibility at the tip for working around the back bone of a fish like a large Halibut. It is much wider at the handle end to provide strength and heft needed to cut through the heavy bones behind the head on a fish like a large stripped bass. The heft also comes in handy for steaking a large fish like a Salmon.
Flexibility is pretty much a matter of preference. I like a medium flex blade knife and will furnish it that way unless the user prefers more flexibility. I do a flat grind to a very thin edge and an even distal taper along the length of the blade from the guard to the tip.