I'm just learning my snake shot too, and have been trying it out on the guys at work. A good snake shot is near-unstoppable precisely because you can reach any part of the goal from a single setup point with very little "tell" as to where you're going to shoot. (Experienced players, please feel free to correct me here, I'd love to learn more too.)
By "width", I assume you mean how far the ball moves laterally before you strike it? This can vary just as much as it does with a pull/push shot - you want to move the ball to where there's a hole in the defense, and slap it in right at that point. However, there's two things you want to keep in mind while moving the ball. First, you don't want to try to move it too far. You can't reliably generate as much push from the pinned position as you can with a push shot, and if you try to move it too far, the ball will slow down and lessen the effect of the shot. At most, you only want to move it from the center to the edge of the goal before hitting it. If there's a gap available, the most effective snake shots only move a player's width to get around a defender.
When you're moving the ball, remember that since it's pinned under your foot to start with, the player will move faster than the ball. So after you push, you'll actually need to pull the rod
back when you start to yank up, to move the player back behind the ball as it strikes. This is what makes the shot go straight, and it ends up being a lot faster than waiting for the ball to arrive at the player's position. It's also the toughest part of the snake shot to coordinate and get used to the feel of. But once you do get used to it, you'll find that your shots actually start connecting.
For speed, the faster the better!
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The goal is to catch the defense with their pants on the ground, so to speak - pick the open hole and put it in before they can react. Practice to the point where the entire shot is one fluid motion. Then make it quick. It has to be done in the blink of an eye, otherwise it won't be long before the defense will get wise and will cover the hole as soon as they see you start to move.