The letters refer to the length and numbers to the girth.
I also wear an F26, but there is no room for my head to retract or to gather that much extra fabric. Even with an girth 28, which I also have for after workouts, I could never gather that much around the silicone ring.
It can be hard to adequately measure because often just touching yourself to get the measurement can cause a small amount of plumping. I know I originally ordered a C30 and had lots of issues until I realized I needed one that was both longer and thinner.
The picture certainly looks like you would need a smaller girth, at the very least for the silicone portion of the innerwear. I would email the support email back and ask for Stealthman, he will help get you sorted.