Atomic size increases with increase in period number.(as we move from above to below in a group )
Atomic size decreases as we move from left to right in a period .
As period number increases , number of shells increase , so atomic size increases .
As we move from left to right in a period , number of electrons in shell increase , so effective nuclear charge ( force of attraction between nucleus of atom which has +ve charge and electrons which have -ve charge) increases so shells are closer to nucleus and atomic size is less .