Identify the element on the periodic table
Use its period number to estimate the number of electron shells
Use its group position to compare size trends within a row
Remember atomic radius generally decreases from left to right across a period
Remember atomic radius generally increases from top to bottom down a group
Compare the element to nearby elements if an exact value is not given
Use the covalent radius, metallic radius, or van der Waals radius depending on the context
For ions, use ionic radius instead of atomic radius
For a quick estimate, larger atoms are usually found toward the bottom-left of the periodic table
For a quick estimate, smaller atoms are usually found toward the top-right of the periodic table