When the moon blocks out the sun during a solar eclipse is it just big enough or does it have extra blocking ability?

Well, they’re not the same size, but the same apparent size. They are both about 30 arcminutes.

http://astronomy.swin.edu.au/cosmos/A/Angular+Diameter

It will depend upon at what point the moon is in its orbit about the earth, and to some extent, at what point the earth is in its orbit about the sun. Both orbits are not perfectly circular. If the moon is at apogee (it’s greatest distance from the earth, and so it will have its smallest apparent size) then the eclipse is likely to be annular. The moon does not completely block the sun, and it appears as bright ring around the moon. At perigee, the moon has its largest apparent size, and so it will completely eclipse the solar disk.

The sun varies in apparent size for the same reason, largest when earth is closest (perihelion) and smallest when earth is farthest away (apihelion).

This is one of the great cosmic coincidences. The sun is 400 times larger than the moon but the moon is 400 times closer than the sun. Thus you have the apparent size of the moon equal to the size of the sun. In a few million years the moon will have drifted farther from earth and will no longer create totality.