You want a rule? Try this one: Wear what you think looks great on you. Really. The real key to fashion is feeling like a million bucks. How you get there is your business.
With that out of the way, we can get down to some practical advice. There was a time when I would have passed along deathless wisdom about matching the blade of the tie to the drop of the collar and the waist of the lapel, but those days are long gone. Most men today dont even know what the blade of their tie is. Instead, remember this: The important thing isn’t the width of the tie, it’s the size of the knot. The knot should fill the space between the wings of your collar without looking crowded. If the knot pushes the wings of and off your chest then it is too big. If there is a gap between the top of the knot and the edges of the collar then it is too small.
The trick to making sure the knot fits just right is all in the knot you choose. Every guy should have two knots in tie toolkit: The half Windsor and the full. If you aren’t sure which is which or how to tie them, go talk to your tailor. He can not only show you these things but also give you a quick rundown on how they should fill your collar space and how to angle the sides of the knot. That leaves you ready to buy almost any tie that catches your eye and still wear it like a pro.
If you are still hung up on widths for some reason you can always go with this: Don’t wear a wide tie if you are a skinny guy. Don’t wear a skinny tie if you are a wide guy. And no matter what size you are, a tie that is 9 cm across at the widest part is going to look good. When push comes to shove you can always fall back on that.