What size fillet knife do I need?

This completely depends on the type of meat or fish that you plan to prepare. Fillet knives have blades from around 6 inches to 9 inches. How large is the fish that you normally catch or buy? Trout, perch, or fish of a similar size will only need a blade size of around 6 inches. Larger fish may require a larger blade between 7 inches and 9 inches.

How do I sharpen the knife?

There are several different methods of knife sharpening and the way in which you would sharpen a knife depends on the type of sharpener that you choose to use. Some people like to use an electric sharpener which makes the job easy for you as you just allow the machine to do its job. A sharpening stone can also be used; this is a type of stone that is used with oil or water. If you have a water stone then you will need to soak it under water first for around 5 minutes and then slowly move the blade from side to side across the stone on both sides with your hands flat against the "safe" part of the blade. There are lots of YouTube videos that demonstrate this beautifully. You can also use a sharpening rod which is used slightly differently. Again, YouTube is a useful source for these videos.

Can I wash the fillet knife in a dishwasher?

Some blades are dishwasher safe but that doesn't mean that you should ever wash them in the dishwasher. Dishwasher detergent is super abrasive and they are likely to get damaged in the machine. The best way to take care of a fillet knife is to hand wash it in hot soapy water, taking care not to cut your hands.