Bow ties ought to be the perfectly sized tie around your collar. It has to fit in perfectly and also fit well with your body proportions. Here is a basic guide that will ensure you get the size right!
The length of the bow-tie will be based on your facial features. There is a perfect length and then there is too long a bow-tie or too narrow a bow-tie. You need to have the perfect length and here is how you can identify at what length the bow-tie is just right!
For any style of bow-tie be it butterfly or diamond, the ends of the bow-tie will be aligned with your eyebrow corners. This is the perfect length because the basic purpose of the bow-tie is to establish strong symmetry which is very appealing. This symmetry can be further accentuated by aligning your bow-tie with your eyebrows. Thus your bow-tie should end where your eyebrow line does.
Even if you go for a casual look, let your bow tie be wider but the length should be precise. A wider bow will essentially look over the top and a narrow bow will appear irrelevant and stupid. We need to avoid both the scenarios.
Now the width of the bow is a concept that you may take liberty with but ensure you do not go overboard with the same. Your bow tie length will be twice its width. The bow tie height or width will be almost three times the knot size because the knot will have to be proportionately small and tight. Also, the knot should be a perfect square if your look is for an all formal event but can be a bit rectangular with longer length than height if for a casual or semi-formal event!