I think you're talking about the black spot on the surface of the Earth,
where the end of the Moon's shadow falls, and anybody inside that
circle sees the Sun completely covered for a few minutes.
The size of that spot varies. It depends on exactly how far the Moon
is from the Earth, and exactly how far both bodies are from the Sun.
The longest possible totality viewed from a single location is about
7-1/2 minutes, when all the geometry is just right. Then, I believe,
the black spot on the Earth's surface is something like 170 miles
across, moving at around 24 miles a minute, or 1,460 miles per hour.