An atom is full of electrons. The atom is like a container you fill with electrons, rather than a solar system-like arrangement. The atom gets larger when you add more electrons. This is because the space around the electrons is already full and the electrons that are needed to find more space need to be found.
This is a simplified description of electrons filling atoms. However, the filled space model is much more accurate than the common solar system-like model.
Reality is stranger than fiction.
The filled space model is based upon the electron being very lightweight, which means that its de Broglie wavelength will be much longer than the nucleus. The de Broglie wavelength can be used as a proxy to the size of an electron.
You can use the de Broglie wavelength to calculate the probability of finding an electron. This will allow you to combine the measurement of position and the size of the element. However, position is only a number. If the electron is a point particle, size and position match up. However, an electron that is infinitely small would be impossible to detect.
It is difficult to reconcile quantum measurements with classical ideas of size. Quantum perspective shows that the atom is full electrons. The atom is empty from a classical perspective because point particles have no volume.
It is conceptually easier to explain the atom as full than it is to explain the workings of an empty atom.