How big must a space rock be to reach the ground as a meteorite?
Typically, a stony meteoroid needs to be at least about the size of a grapefruit to a football, with a few kilograms of mass, to leave pieces on the ground. Slow, strong ones survive best.
Most meteoroids burn up completely. For something to survive as a meteorite, it needs enough mass to lose most of its outer material and still have something left, and it needs to slow down before it is destroyed.
Rough size. Stony meteoroids smaller than a few centimetres usually vaporise. Those of perhaps 10 to 20 centimetres across, weighing a few kilograms or more, can drop small meteorites if they enter at a moderate speed and angle. Larger meteoroids, a metre or more across, often produce many fragments over a strewn field. Typically only a small fraction of the original mass reaches the ground, sometimes just a few per cent or less.
Speed. Slow meteoroids, around 12 to 20 kilometres per second, survive much better than fast ones. Above about 25 to 30 kilometres per second, very little survives, which is one reason why meteor showers from comets, with fast particles, do not drop meteorites.
Strength. Iron meteoroids are much stronger than stony ones and survive better. Fragile carbonaceous meteoroids often break apart high up.
Angle. Steep entries pass through the atmosphere quickly, while shallow ones spend longer heating, which can either help them slow gently or destroy them.
Tiny exceptions. Very small grains, under about a millimetre, can drift down slowly without melting completely, becoming micrometeorites. So the smallest and some of the larger space rocks survive, while those in between usually vanish as meteors.