The proper answer is: it depends on the element, but the atom’s diameter is in the range 0.1–0.5 nanometres .
But proper answers don’t always give you an idea of what something looks like in real life, right?
Let’s try again: an atom is a million times smaller than the thickest human hair. They are so small that, if we inflated all the atoms in a grapefruit to the size of a blueberry, the grapefruit would be as big as Earth! You don’t believe me?! Check this: