How do I find out which channel size I need in my seat? I am male and want what is the healthiest choice.
Pretty much all of the cutouts in modern seats will achieve the desired effect (less pressure on the nerves that run through the crotch area), so it comes down to what is most comfortable for you.
There are quite a few different models out there, such as ISM Adomo, Cobb saddles, Fizik, etc. Many people that are quite comfortable on one are not comfortable on another. Your best bet is to find a cycling or triathlon shop that has a "try and swap" arrangement, i.e. if you try it for a couple weeks and find it really uncomfortable, you can swap it for a different model/brand.
This will depend on what kind of bike you have, and what kind of riding you do. Are you a commuter? If so you may want a wide, padded seat, and a wide channel. If you are a sprint racer you may not need a channel at all, as you will be standing so you just want a narrow seat.
At the end of the day, try some and see what is comfortable - everyone is different. Mine has a negligible channel, but the shape of the seat is such that my 16 mile commute to work is still comfortable.