How do I find out which channel size I need in my seat?
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.