The size of an atom depends upon its atomic number and especially the number of energy levels present. To answer this question, let's consider the smallest atom, hydrogen.
A hydrogen atom consists of a single proton and a single electron. If the single proton was the size of a ping pong ball (about 1.5 inches (40 mm)) and at the center of the atom, then the farthest expected distance we would ever expect to find the electron would be about 4,040 ft away (about 13-1/2 football fields away.)
The diameter of an electron is ~1/1000 of a proton, and so in our scale model, the electron would be about 1/1000th of an inch (about the thickness of a hair.)
The geometry and ratios of sizes in atoms is indeed strange. In between the proton and the electron would be empty space (literally nothing). Larger atoms would be significantly larger than this hydrogen atom.
