If a black hole is the size of a grain of sand, would it still be considered a black hole?

 The singularity of a black hole is absurdly smaller than a grain of sand, so yes.

The singularity is essentially the center of the black hole; it's where matter is drawn in with no possibility of escape.

The rest of what we "see" of a black hole, the accretion disk (the part that emits light), is just the event horizon. In reality, it's ridiculously far from the singularity in human terms. Especially if we're talking about supermassive black holes, where the distance is several million kilometers...

The singularity is of zero size in classical physics, or something relative to the Planck length. But the gravitational pull is so brutal that the accretion disk can be thousands of times the size of a star...

To give you an idea, in a supermassive black hole, the distance between the accretion disk and the singularity would be absurdly greater than the distance between the Earth and the core of the sun.

That's the scale we're talking about...