That is why I like the Dawson front sights, they have a recession/counterbore on the rear of the blade (the side you look at when aiming) that provides a consistent size dot. It makes melting the fiber so much easier when changing it out. The front of the sight dot size doesn't matter as you don't see it (as long as it is big enough to not pull through).
In my experience if you leave about 1/16 of an inch exposed on either side of the sight it will result in a good size dot. You just have to practice how close to get the flame as mentioned already.
Cheers,
Toby