Also depends on width at or around the knot and all the way down. Too skinny up top looks wrong no matter what the width at tip. 6cm is ok if its straight all the way up/down - 6 cm at bottom and thinner up top looks unbalanced. If you are wearing a jacket mostly buttoned up then the bottom width doesn't matter much at all - no one will see it. If not wearing a jacket in summer then a 6.5 - 7 cm almost straight all the way down knit tie is preferable. I'm embarrassed to admit this but a few weeks ago I counted my ties, and re-reorganised and tossed a few, they range from 9cm to 6cm. I'm even more embarrassed to admit that the other day, despite my best educational efforts and despite a generally good record, the Ms mused aloud that she thought very thin black leather ties looked ok.