As atomic no. increases proton no. & 1 electron increases in valence shell but no. of valence shell remains same increase in effective nuclear charge is there as a result attraction of valence electron in valence shell & atomic nucleus progressively smaller from Na to Cl in a period...
As per to the Modern Periodic Table , Atomic Size of the Elements across the period gets decreased because no new shell is added to the elements when we go across the periods .

Whereas , going down the group atomic size mutually increases due to addition of new shell in the elements down the group .
1 4 1