The size of an atom can be estimated by measuring the distance between adjacent atoms in a covalent compound. The covalent radius of a chlorine atom, for example, is half the distance between the nuclei of the atoms in a Cl₂ molecule.

The covalent radii of the main group elements are given in the figure below. These data confirm the trends observed for metallic radii. Atoms become larger as we go down a column of the periodic table, and they becomes smaller as we go across a row of the table.

The covalent radius for an element is usually a little smaller than the metallic radius. This can be explained by noting that covalent bonds tend to squeeze the atoms together, as shown in the figure below.

Information provided by: http://chemed.chem.purdue.edu