If I wanted to destroy the earth, and I mean fucking shatter it, what is the minimum size...

is it feasible that there are asteroids flying around at 99% C?

no

Objects passing close to black holes can fuck off with some decent percentage of C.

The Three Body Problem by Cixin Liu

Don't spoil it. I've only read the first book

First book I've enjoyed in a long time.

The Asteroid would need to be:

3,356,158 meters wide (3356.158km)
Be made of iron having a density of 8000 kg/m^3
Traveling 30km/s (67,000 mph)
Hit Earth at a 90 degree angle head on (Earth is traveling 30km/s too)

The result would be an asteroid belt between Venus and Mars. Anything less and the Earth will not overcome its own gravity, but would be a completely melted mass which would cool and form another planet.

Oh and it would need to hit sedimentary rock on land somewhere, not ocean or crystalline rock.

Would crashing the moon into the earth have the same effect?

No, it isn't large enough or dense enough for that speed to affect Earth enough to break Earth apart. Remember, the moon is actually a piece of something Mars sized that hit the Earth long ago. Now there's far less mass to it than it originally had.

It would need to travel as a much faster speed than 30km/s.