We will use their positions in the periodic tbale and the perioidic trends . We notice that P and S are in the same row , row 3rd of the periodic table .Therefore,we expect the radius of S to be smaller than that of P because radii decreases as we move from left to right .
Likewise,the radius of Se is expected to be less than that of d.We also notice that As is directly below P and that Se is directly below S .We expect that the radius of As is greater than that of P and that the radius of Se is greaater than that of S radii increases as we move from top to bottom .
Based on these observations we can conclude that S has the smallest radius of four elements and that As has the largest radius.
Its hard to decide weather P or Se has the larger radius , if we move from P to Se radius tend to increase as you move down and radius tend to decrease as you move to the right however radius of Se (1.117 A is greater then that of P 1.10 A .
S< P < Se < As