One trend that is studied for atomic systems in the "atomic size". The size of an atom is difficult to define. This is because the wavefunction of an electron in an atom extends to infinity. However at some distance away from the nucleus the probability of finding the electron while finite is essentially zero. However, this "size" is chosen rather than being something that can be directly measured. Alternatively, atomic size can be determined based on experimental data. For example, the size of a chlorine atom might be deduced from the distance between two chlorine atoms in molecular chlorine, Cl2. The size of ions (atoms with a charge from either excess or missing electrons) could be deduced from the distances between the nuclei in a crystalline solid.
Since all atoms and ions are spherical, there size is typically given as a radius. The size of an atom (ion) is referred to as its atomic (ionic) radius. As noted above this radius could be determined in a number of ways. Therefore the absolute numbers you might find in a table can vary from source to source. However, the trends in atomic radii should hold regardless of how the radius is defined.
Atomic radii are related to the "radial distribution function" we saw for the wavefunctions (atomic orbitals). Those told us the probability of finding an electron at a particular radius. The trend for atomic radii will hold for the most probable distance from the radial distribution function with the caveat that atomic radii are typically derived from the highest energy valence electrons.
Finally, despite the trends atomic radii are all quite surprisingly similar. The balance of adding both protons and electrons to different elements helps to generally balance the interactions in the atoms. Thus even the largest of atoms like cesium at 2.98 x 10-10 m while the smallest helium is 3.1 x 10-11 m. This is a difference of only a factor of ten. The vast majority of the atoms have a size that is about 10-10 m. This unit of length has its own name, an Ångstrom, which is defined as 1 Å is equal to 1 x 10-10 m (0.1 nm). This is the "size of a typical" atom.