Question

why does the atomic size increases down the group while decreases along the period..?

Answer 1

down the group there is increase in atomic size in terms of energy levels 
this causes the outermost electron to be loosely attached to the nucleaur
across the period there is an increase in atomic forces which strongly pulls or attaracts the outermost electrons closer to the nuleus   hence reducing the radius

Answer 2

as the number of electrons increase while going down the group, the atomic size of the atom increases
whereas while going along the period, the attractive force becomes more and thereby the atomic size decreases
Hope It Helps!!