Sodium has a larger atomic radius than lithium, as it contains more electrons. Atomic radius is determined by the total number of electrons an atom has. A higher number of electrons equals a larger atomic radius.
On the periodic table, atomic radius increases both from left to right and from top to bottom. The first element on the table, hydrogen, has the lowest atomic radius, while those in the bottom-right corner have the highest radius.
Both lithium and sodium are members of the alkali metals group, which also includes potassium, rubidium, cesium and francium. Of these elements, lithium has the smallest atomic radius and francium has the largest.