A large body doesn't really matter, but the small head does. It's about half the size (linear measurements) of a deer head for reference, which is a pretty easy

A large body doesn't really matter, but the small head does. It's about half the size (linear measurements) of a deer head for reference, which is a pretty easy shot for almost anyone at 50 yards and not bad with a twidge of focused training at 250 yards. However, for whatever reason, snakes don't seem to die very quickly when you shoot them in the head a few times (enough news stories if you're interested). I'd imagine that's at least part of where the recommendation comes from. Plus, they die incredibly slowly (if ever) if you go for a body shot. It'd probably suffer less if you drowned it (an ordeal that itself also takes hours), and none of those methods are even slightly humane.