Question

The size of sodium is smaller than neutral sodium, why?

Answer 1

For the same reason, positive ions should be smaller than the atoms from which they are formed. The 11 protons in the nucleus of an Na+ ion, for example, should be able to hold the 10 electrons on this ion more tightly thanthe 11 electrons on a neutral sodium atom.
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Answer 2

coz sodium ion is Na+ (Na atomic no.-11,so electric configuration 2,8,1.so to be stable it will donate one electron and positive sign will come.) which makes it small compared to neutral one coz it doesn't donate any electron.